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matplotlib & pandas
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Python/matplotlab/advanced/path_tutorial.md
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Python/matplotlab/advanced/path_tutorial.md
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---
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sidebarDepth: 3
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sidebar: auto
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---
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# Path Tutorial
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Defining paths in your Matplotlib visualization.
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The object underlying all of the ``matplotlib.patch`` objects is
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the [``Path``](https://matplotlib.orgapi/path_api.html#matplotlib.path.Path), which supports the standard set of
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moveto, lineto, curveto commands to draw simple and compound outlines
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consisting of line segments and splines. The ``Path`` is instantiated
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with a (N,2) array of (x,y) vertices, and a N-length array of path
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codes. For example to draw the unit rectangle from (0,0) to (1,1), we
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could use this code
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``` python
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import matplotlib.pyplot as plt
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from matplotlib.path import Path
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import matplotlib.patches as patches
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verts = [
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(0., 0.), # left, bottom
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(0., 1.), # left, top
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(1., 1.), # right, top
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(1., 0.), # right, bottom
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(0., 0.), # ignored
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]
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codes = [
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Path.MOVETO,
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Path.LINETO,
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Path.LINETO,
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Path.LINETO,
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Path.CLOSEPOLY,
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]
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path = Path(verts, codes)
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fig, ax = plt.subplots()
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patch = patches.PathPatch(path, facecolor='orange', lw=2)
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ax.add_patch(patch)
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ax.set_xlim(-2, 2)
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ax.set_ylim(-2, 2)
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plt.show()
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```
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The following path codes are recognized
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---
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Code
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Vertices
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Description
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STOP
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1 (ignored)
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A marker for the end of the entire path (currently not required and ignored)
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MOVETO
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1
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Pick up the pen and move to the given vertex.
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LINETO
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1
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Draw a line from the current position to the given vertex.
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CURVE3
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2 (1 control point, 1 endpoint)
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Draw a quadratic Bézier curve from the current position, with the given control point, to the given end point.
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CURVE4
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3 (2 control points, 1 endpoint)
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Draw a cubic Bézier curve from the current position, with the given control points, to the given end point.
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CLOSEPOLY
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1 (point itself is ignored)
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Draw a line segment to the start point of the current polyline.
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## Bézier example
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Some of the path components require multiple vertices to specify them:
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for example CURVE 3 is a [bézier](https://en.wikipedia.org/wiki/B%C3%A9zier_curve) curve with one
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control point and one end point, and CURVE4 has three vertices for the
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two control points and the end point. The example below shows a
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CURVE4 Bézier spline -- the bézier curve will be contained in the
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convex hull of the start point, the two control points, and the end
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point
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``` python
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verts = [
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(0., 0.), # P0
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(0.2, 1.), # P1
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(1., 0.8), # P2
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(0.8, 0.), # P3
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]
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codes = [
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Path.MOVETO,
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Path.CURVE4,
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Path.CURVE4,
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Path.CURVE4,
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]
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path = Path(verts, codes)
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fig, ax = plt.subplots()
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patch = patches.PathPatch(path, facecolor='none', lw=2)
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ax.add_patch(patch)
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xs, ys = zip(*verts)
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ax.plot(xs, ys, 'x--', lw=2, color='black', ms=10)
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ax.text(-0.05, -0.05, 'P0')
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ax.text(0.15, 1.05, 'P1')
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ax.text(1.05, 0.85, 'P2')
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ax.text(0.85, -0.05, 'P3')
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ax.set_xlim(-0.1, 1.1)
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ax.set_ylim(-0.1, 1.1)
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plt.show()
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```
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## Compound paths
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All of the simple patch primitives in matplotlib, Rectangle, Circle,
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Polygon, etc, are implemented with simple path. Plotting functions
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like [``hist()``](https://matplotlib.orgapi/_as_gen/matplotlib.axes.Axes.hist.html#matplotlib.axes.Axes.hist) and
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[``bar()``](https://matplotlib.orgapi/_as_gen/matplotlib.axes.Axes.bar.html#matplotlib.axes.Axes.bar), which create a number of
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primitives, e.g., a bunch of Rectangles, can usually be implemented more
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efficiently using a compound path. The reason ``bar`` creates a list
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of rectangles and not a compound path is largely historical: the
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[``Path``](https://matplotlib.orgapi/path_api.html#matplotlib.path.Path) code is comparatively new and ``bar``
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predates it. While we could change it now, it would break old code,
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so here we will cover how to create compound paths, replacing the
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functionality in bar, in case you need to do so in your own code for
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efficiency reasons, e.g., you are creating an animated bar plot.
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We will make the histogram chart by creating a series of rectangles
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for each histogram bar: the rectangle width is the bin width and the
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rectangle height is the number of datapoints in that bin. First we'll
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create some random normally distributed data and compute the
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histogram. Because numpy returns the bin edges and not centers, the
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length of ``bins`` is 1 greater than the length of ``n`` in the
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example below:
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``` python
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# histogram our data with numpy
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data = np.random.randn(1000)
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n, bins = np.histogram(data, 100)
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```
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We'll now extract the corners of the rectangles. Each of the
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``left``, ``bottom``, etc, arrays below is ``len(n)``, where ``n`` is
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the array of counts for each histogram bar:
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``` python
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# get the corners of the rectangles for the histogram
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left = np.array(bins[:-1])
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right = np.array(bins[1:])
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bottom = np.zeros(len(left))
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top = bottom + n
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```
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Now we have to construct our compound path, which will consist of a
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series of ``MOVETO``, ``LINETO`` and ``CLOSEPOLY`` for each rectangle.
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For each rectangle, we need 5 vertices: 1 for the ``MOVETO``, 3 for
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the ``LINETO``, and 1 for the ``CLOSEPOLY``. As indicated in the
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table above, the vertex for the closepoly is ignored but we still need
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it to keep the codes aligned with the vertices:
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``` python
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nverts = nrects*(1+3+1)
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verts = np.zeros((nverts, 2))
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codes = np.ones(nverts, int) * path.Path.LINETO
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codes[0::5] = path.Path.MOVETO
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codes[4::5] = path.Path.CLOSEPOLY
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verts[0::5,0] = left
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verts[0::5,1] = bottom
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verts[1::5,0] = left
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verts[1::5,1] = top
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verts[2::5,0] = right
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verts[2::5,1] = top
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verts[3::5,0] = right
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verts[3::5,1] = bottom
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```
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All that remains is to create the path, attach it to a
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``PathPatch``, and add it to our axes:
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``` python
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barpath = path.Path(verts, codes)
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patch = patches.PathPatch(barpath, facecolor='green',
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edgecolor='yellow', alpha=0.5)
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ax.add_patch(patch)
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```
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``` python
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import numpy as np
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import matplotlib.patches as patches
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import matplotlib.path as path
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fig, ax = plt.subplots()
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# Fixing random state for reproducibility
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np.random.seed(19680801)
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# histogram our data with numpy
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data = np.random.randn(1000)
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n, bins = np.histogram(data, 100)
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# get the corners of the rectangles for the histogram
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left = np.array(bins[:-1])
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right = np.array(bins[1:])
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bottom = np.zeros(len(left))
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top = bottom + n
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nrects = len(left)
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nverts = nrects*(1+3+1)
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verts = np.zeros((nverts, 2))
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codes = np.ones(nverts, int) * path.Path.LINETO
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codes[0::5] = path.Path.MOVETO
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codes[4::5] = path.Path.CLOSEPOLY
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verts[0::5, 0] = left
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verts[0::5, 1] = bottom
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verts[1::5, 0] = left
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verts[1::5, 1] = top
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verts[2::5, 0] = right
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verts[2::5, 1] = top
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verts[3::5, 0] = right
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verts[3::5, 1] = bottom
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barpath = path.Path(verts, codes)
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patch = patches.PathPatch(barpath, facecolor='green',
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edgecolor='yellow', alpha=0.5)
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ax.add_patch(patch)
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ax.set_xlim(left[0], right[-1])
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ax.set_ylim(bottom.min(), top.max())
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plt.show()
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```
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## Download
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- [Download Python source code: path_tutorial.py](https://matplotlib.org/_downloads/ec90dd07bc241d860eb972db796c96bc/path_tutorial.py)
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- [Download Jupyter notebook: path_tutorial.ipynb](https://matplotlib.org/_downloads/da8cacf827800cc7398495a527da865d/path_tutorial.ipynb)
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