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Plotting with Pyplot

The module matplotlib.pyplot (docs) provides functions to add lines, text, images, histograms and more to a plotting area. This is the set of functions used most of the time. We look at pyplot.plot and shortly at pyplot.text. The full list can be found at the pyplot summary. Some examples are:

• arrow: add an arrow to the Axes
• axes: add an Axes to the figure
• axhline, axhspan: add a horizontal line/rectangle across the axis
• figure: creates a new figure
• fill: plot filled polygons
• grid: turn the Axes grids on or off
• hist: plot a histogram
• imshow: display an image on the Axes
• legend: place a legend on the current Axes
• plot: plot lines and/or markers to the Axes
• table: add a table to the current Axes
• text: add text to the Axes
• title: et a title of the current Axes

More information at:

• Pyplot Tutorial

The plot Command

The function pyplot.plot (docs) plots line segments connecting points given as parameter. Matplotlib works internally with numpy arrays, which should be used in case of big arrays. The function accepts a multitude of parameters. If x and y are one dimensional arrays of the same length and m is a two dimensional array having as many rows as x has entries, then some examples are:

• plot(x) will plot the line segments connecting the points (i, x[i])
• plot(x, y) will plot the line segments connecting the points (x[i], y[i])
• plot(x, m) will plot a line for each column where the j-th line is given by the segments connecting the points (x[i], m[i][j])
• plot(x0 , y0, x1, y1) with len(x0) == len(y0) and len(x1) == len(y1), will plot two lines as expected

Additionally plot accepts formatting strings and a list of line properties:

• plot(x, y, 'bo--') will plot a blue (b) dashed line (--), with the points (x[i], y[i]) marked by circles (o)
• plot(x, y, color='green', label='green line', linewidth=2) will plot a green line of width two. The label will be used if a legend is displayed by a subsequent call of legend().

Below a possibly ugly combination of line properties :

# (Image above)
import matplotlib.pyplot as plt
plt.plot([1,2,3,4],[1,4,9,16], 'gD-.', label='green line',\
         linewidth=8, markerfacecolor='yellow', markersize=15)
plt.legend()
plt.show()