plt.savefig(): ValueError: All values in the dash list must be positive

Question

Running the code at the below link results in the error. As far as having something to do with the image, I don't know what the 'dash list' is.

matplotlib.pyplot as plt
...
plt.savefig('tutorial10.png',dpi=300)

Segment of the returned error:

    ---------------------------------------------------------------------------
    ValueError                                Traceback (most recent call last)
    <ipython-input-21-edce1701d7a3> in <module>()
    60     ax.add_collection(lines)
    61 
--> 62 plt.savefig('tutorial10.png',dpi=300)
    63 
    64 plt.show()

    ...

    C:\Anaconda\lib\site-packages\matplotlib\backend_bases.pyc in set_dashes(self, dash_offset, dash_list)
    902             dl = np.asarray(dash_list)
    903             if np.any(dl <= 0.0):
--> 904                 raise ValueError("All values in the dash list must be positive")
    905         self._dashes = dash_offset, dash_list
    906 

http://www.geophysique.be/2013/02/12/matplotlib-basemap-tutorial-10-shapefiles-unleached-continued/

Answer 1

In the code, you linked, there are the following lines:

m.drawparallels(np.arange(y1,y2,2.),labels=[1,0,0,0],color='black',dashes=[1,0],labelstyle='+/-',linewidth=0.2) # draw parallels
m.drawmeridians(np.arange(x1,x2,2.),labels=[0,0,0,1],color='black',dashes=[1,0],labelstyle='+/-',linewidth=0.2)

In those lines the argument dashes is set to [1,0]. Concerning to your error message, all values in the array dashes must be strictly positive. That's why you get the exception (your array dashes contains zero).

Answer 2

In matplotlib 3.6.2, this error can also appear when plotting a dashed line with a null line width:plt.plot(1, 1, ls='--', lw=0)