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I'd like to plot numerical data against non numerical data, say something like this:

import matplotlib.pyplot as pl
x=['a','b','c','d']
y=[1,2,3,4]
pl.plot(x,y)

However, with matplotlib plot packages you get a warning that the data is not float (ValueError: invalid literal for float(): a).

In their 'How-to', they suggest to put first the numerical data on the x axis and then format it. Is there a way to do it directly (as above)?

Max Li
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2 Answers2

15

Use the xticks function.

import matplotlib.pyplot as pl
xticks=['a','b','c','d']
x=[1,2,3,4]
y=[1,2,3,4]
pl.plot(x,y)
pl.xticks(x,xticks)
pl.show()
Stephen Terry
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13
import matplotlib.pyplot as plt
x = ['a','b','c','d']
y = [1,2,3,4]
plt.plot(y)
plt.xticks(range(len(x)), x)
plt.show()

enter image description here

On a side note, dates are numerical in this sense (i.e. they have an inherent order and spacing).

Matplotlib handles plotting temporal data quite well and very differently than the above example. There's an example in the matplotlib examples section, but it focuses on showing off several different things. In general, you just use either plot_date or just plot the data and call ax.xaxis_date() (or yaxis_date) to tell matplotlib to use the various date locators and tickers.

Joe Kington
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