Alexander Yau Alexander Yau - 12 days ago 6
Python Question

matplotlib.pyplot.plot, ValueError: could not convert string to float: A

I want to plot the bar plot, here is the data:

large_letter = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J']
large_num = [52909, 52911, 52912, 52911, 52912, 52912, 52912, 52912, 52912, 52911]
small_letter = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J']
small_num = [1872, 1873, 1873, 1873, 1873, 1872, 1872, 1872, 1872, 1872]


And I want to plot 2 subplots to show each letters' number in each list, so here is my code

import matplotlib.pyplot as plt
fig, (ax1, ax2) = plt.subplots(1, 2)
ax1.bar(large_letter, large_num)
ax2.bar(small_letter, small_num)


But it returned the following error:

ValueErrorTraceback (most recent call last)
<ipython-input-90-e23f798dd50c> in <module>()
20 fig, (ax1, ax2) = plt.subplots(1, 2)
---> 21 ax1.bar(large_letter, large_num)
22 ax2.bar(small_letter, small_num)

/usr/lib64/python2.7/site-packages/matplotlib/__init__.pyc in inner(ax, *args, **kwargs)
1817 warnings.warn(msg % (label_namer, func.__name__),
1818 RuntimeWarning, stacklevel=2)
-> 1819 return func(ax, *args, **kwargs)
1820 pre_doc = inner.__doc__
1821 if pre_doc is None:

/usr/lib64/python2.7/site-packages/matplotlib/axes/_axes.pyc in bar(self, left, height, width, bottom, **kwargs)
2085 edgecolor=e,
2086 linewidth=lw,
-> 2087 label='_nolegend_'
2088 )
2089 r.update(kwargs)

/usr/lib64/python2.7/site-packages/matplotlib/patches.pyc in __init__(self, xy, width, height, angle, **kwargs)
638 Patch.__init__(self, **kwargs)
639
--> 640 self._x = float(xy[0])
641 self._y = float(xy[1])
642 self._width = float(width)

ValueError: could not convert string to float: A


How can I fix this problem? Thank you!

Answer

It tells you exactly what the problem is -- it's trying to convert the first argument to a number. Have a look at the documentation for matplotlib.pyplot.bar. It's a bit unintuitive, but the first argument is a coordinate for the left side of the bar, not the label of the bar. I'm not sure, but you probably needing the tick_label argument to name the bars.

Try something like

# Specify the width of each of the bars since we need it for calculating left of
# bars. The default is 0.8
width = 1.0

# Find the lefts of the bars. You can change this to leave a gap between bars
lefts = [x * width for x, _ in enumerate(large_num)]

# Create bar graph
ax1.bar(lefts, large_num, width=width, tick_label=large_letter)