pyplot: Add graph to existing plot

Question

In order to draw two graphs in one plot with python's matplotlib.pyplot, one would typically do this:

import matplotlib.pyplot as plt

xdata = [0, 1, 2, 3, 4]
ydata1 = [0, 1, 2, 3, 4]
ydata2 = [0, 0.5, 2, 4.5, 8]

plt.plot(xdata, ydata1, 'r-', xdata, ydata2, 'b--')
plt.show()

However, I would like to draw the second dataset only under certain circumstances, like this:

plt.plot(xdata, ydata1, 'r-')

if DrawSecondDataset:
    plt.plot(data, ydata2, 'b--')

Unfortunately, calling plot for the second time means that the first dataset is erased.

How can you add a graph to an already existing plot?

EDIT:

As the answers pointed out correctly, the dataset is only erased if plt.show() has been called between the two plt.plot() commands. Thus, the example above actually shows both datasets.

For completeness: Is there an option to add a graph to an existing plot on which plt.show() has already been called? Such as

plt.plot(xdata, ydata1, 'r-')
plt.show()

...

plt.plot(data, ydata2, 'b--')
plt.show()

"Unfortunately, calling plot for the second time means that the first dataset is erased" No it doesn't. — Stop harming Monica
– Stop harming Monica, Commented Dec 22, 2016 at 17:40
Yes, you are right. It only is when you call show in between. I have updated my question. — Julian Helfferich
– Julian Helfferich, Commented Dec 23, 2016 at 6:34

nick_name · Accepted Answer · 2016-12-22 16:34:16Z

2

Just call show() at the very end.

answered Dec 22, 2016 at 16:34

nick_name

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2 Comments

Julian Helfferich Over a year ago

Yes, that works. But only if you haven't called show before. Do you also know how to add a graph after show has been called.

Julian Helfferich Over a year ago

To answer my own question: After you have called show, the next plot command will clear the plot. Then, you can again call plot twice to add two plots. Thank you very much.

Taufiq Rahman · Accepted Answer · 2016-12-22 16:37:34Z

0

if DrawSecondDataset:
    plt.plot(data, ydata2, 'b--')
    plt.show() #to display it

answered Dec 22, 2016 at 16:37

Taufiq Rahman

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Comments

Alejo Bernardin · Accepted Answer · 2016-12-22 16:46:18Z

0

The first data won't be erased in this way

plt.plot(xdata, ydata1, 'r-')

if DrawSecondDataset:
    plt.plot(data, ydata2, 'b--')

plt.show()

answered Dec 22, 2016 at 16:46

Alejo Bernardin

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