Defining paths in your Matplotlib Visualization.
The object underlying all of the matplotlib.patch objects is the Path, which supports the standard set of moveto, lineto, curveto commands to draw simple and compound outlines consisting of line segments and splines. The Path is instantiated with an (N,2) array of (x,y) vertices, and an N-length array of path codes. For example, to draw the unit rectangle from (0,0) to (1,1), we could use this code.
import matplotlib.pyplot as plt from matplotlib.path import Path import matplotlib.patches as patches verts = [ (0., 0.), # left, bottom (0., 1.), # left, top (1., 1.), # right, top (1., 0.), # right, bottom (0., 0.), # ignored ] codes = [ Path.MOVETO, Path.LINETO, Path.LINETO, Path.LINETO, Path.CLOSEPOLY, ] path = Path(verts, codes) fig, ax = plt.subplots() patch = patches.PathPatch(path, facecolor='red', lw=2) ax.add_patch(patch) ax.set_xlim(-2, 2) ax.set_ylim(-2, 2) plt.show()
Some of the path components require multiple vertices to specify them: for example CURVE 3 is a bézier curve with one control point and one endpoint, and CURVE4 has three vertices for the two control points and the endpoint. The example below shows a CURVE4 Bézier spline -- the bézier curve will be contained in the convex hull of the start point, the two control points, and the endpoint.
verts = [ (0., 0.), # P0 (0.2, 1.), # P1 (1., 0.8), # P2 (0.8, 0.), # P3 ] codes = [ Path.MOVETO, Path.CURVE4, Path.CURVE4, Path.CURVE4, ] path = Path(verts, codes) fig, ax = plt.subplots() patch = patches.PathPatch(path, facecolor='none', lw=2) ax.add_patch(patch) xs, ys = zip(*verts) ax.plot(xs, ys, 'x--', lw=2, color='black', ms=10) ax.text(-0.05, -0.05, 'P0') ax.text(0.15, 1.05, 'P1') ax.text(1.05, 0.85, 'P2') ax.text(0.85, -0.05, 'P3') ax.set_xlim(-0.1, 1.1) ax.set_ylim(-0.1, 1.1) plt.show()
All of the simple patch primitives in matplotlib, Rectangle, Circle, Polygon, etc, are implemented with a simple path. Plotting functions like hist() and bar(), which create a number of primitives, e.g., a bunch of Rectangles, can usually be implemented more efficiently using a compound path. The reason bar creates a list of rectangles and not a compound path is largely historical: the Path code is comparatively new and the bar predates it. While we could change it now, it would break old code, so here we will cover how to create compound paths, replacing the functionality in the bar, in case you need to do so in your own code for efficiency reasons, e.g., you are creating an animated bar plot.
We will make the histogram chart by creating a series of rectangles for each histogram bar: the rectangle width is the bin width and the rectangle height is the number of data points in that bin. First, we'll create some random normally distributed data and compute the histogram. Because numpy returns the bin edges and not centers, the length of bins is 1 greater than the length of n in the example below:
import numpy as np import matplotlib.patches as patches import matplotlib.path as path fig, ax = plt.subplots() # Fixing random state for reproducibility np.random.seed(19680801) # histogram our data with numpy data = np.random.randn(1000) n, bins = np.histogram(data, 100) # get the corners of the rectangles for the histogram left = np.array(bins[:-1]) right = np.array(bins[1:]) bottom = np.zeros(len(left)) top = bottom + n nrects = len(left) nverts = nrects*(1+3+1) verts = np.zeros((nverts, 2)) codes = np.ones(nverts, int) * path.Path.LINETO codes[0::5] = path.Path.MOVETO codes[4::5] = path.Path.CLOSEPOLY verts[0::5, 0] = left verts[0::5, 1] = bottom verts[1::5, 0] = left verts[1::5, 1] = top verts[2::5, 0] = right verts[2::5, 1] = top verts[3::5, 0] = right verts[3::5, 1] = bottom barpath = path.Path(verts, codes) patch = patches.PathPatch(barpath, facecolor='green', edgecolor='black', alpha=0.5) ax.add_patch(patch) ax.set_xlim(left, right[-1]) ax.set_ylim(bottom.min(), top.max()) plt.show()
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