Python: Plotting strings on x axis of matplot scatterplot yields "could not convert string to float"

Question

I have a dictionary aggregate_dict that has letters a:z as keys, with each key having a list containing exactly 11 numerical values between 0 and 1 as its associated value. I wish to use this data to create a scatter plot with the letters in the alphabet arranged from a to z on the x axis, and each of the values for the given letter on the y axis, like this:

enter image description here

I normally hash this out in R in a second, but I don't have R on the machine I'm working on and I don't have admin rights, so I'm left to do it in Python. I'm trying to go about this in the following way (with simplified dict below):

from matplotlib import pyplot

def scatterplot(x,y):
    pyplot.plot(x,y,'b.')
    pyplot.ylim(0,1)
    pyplot.title("Relative Frequency of Letters")
    pyplot.show()

x_vals = []
y_vals = []

aggregate_dict = {'a':[.1,.2,.3],'b':[.2,.3,.4]}

for u in aggregate_dict:
    x_vals.append(u)
    y_vals.append(aggregate_dict[u])

scatterplot( x_vals, y_vals )

Unfortunately, this approach yields the following trace:

Traceback (most recent call last):
  File "C:/Users/dduhaime/Desktop/letter_distributions_each_section_tale.py", line 74, in <module>
    scatterplot(x_vals, y_vals )
  File "C:/Users/dduhaime/Desktop/letter_distributions_each_section_tale.py", line 56, in scatterplot
    pyplot.plot(x,y,'b.')
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\pyplot.py", line 2987, in plot
    ret = ax.plot(*args, **kwargs)
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\axes.py", line 4138, in plot
    self.add_line(line)
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\axes.py", line 1497, in add_line
    self._update_line_limits(line)
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\axes.py", line 1508, in _update_line_limits
    path = line.get_path()
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\lines.py", line 743, in get_path
    self.recache()
  File "C:\Python27\ArcGIS10.2\lib\site-packages\matplotlib\lines.py", line 420, in recache
    x = np.asarray(xconv, np.float_)
  File "C:\Python27\ArcGIS10.2\lib\site-packages\numpy\core\numeric.py", line 320, in asarray
    return array(a, dtype, copy=False, order=order)
ValueError: could not convert string to float: a

Does anyone know how I might be able to resolve this error and carry on with my plot? I would be grateful for any advice you can offer!

tacaswell · Accepted Answer · 2014-09-02 19:10:05Z

3

import matplotlib.pyplot as plt
from matplotlib.ticker import FuncFormatter, MultipleLocator

x_data = np.arange(97, 110)
y_data = np.random.rand(len(x_data))

def ord_to_char(v, p=None):
    return chr(int(v))

fig, ax = plt.subplots()
ax.plot(x_data, y_data, 'x')
ax.xaxis.set_major_formatter(FuncFormatter(ord_to_char))
ax.xaxis.set_major_locator(MultipleLocator(1))
plt.show()

Relevant doc: http://matplotlib.org/api/ticker_api.html#module-matplotlib.ticker

answered Sep 2, 2014 at 19:10

tacaswell

88.2k23 gold badges222 silver badges203 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

duhaime Over a year ago

excellent @tcaswell! Is there a way to plot multiple values for each tick or value on the x-axis, so that there will be 3+ values for each letter on the x axis?

tacaswell Over a year ago

plot puts a point at every (x, y) pair, just tripple up x values (so it looks like x = [97, 97, 97, 98, 98, 98] and y = range(6)

duhaime Over a year ago

perfect--got me there! I didn't realize how easy it is to convert str to int with ord()! What a useful method! Thank you!

tacaswell Over a year ago

Sorry, should have included that detail

Collectives™ on Stack Overflow

Python: Plotting strings on x axis of matplot scatterplot yields "could not convert string to float"

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related