Font is causing trouble in Linux cronjobs #38

If I use your calmap function on my Linux Red Hat Enterprise distribution, I'll get the warning
findfont: Font family ['Arial'] not found. Falling back to DejaVu Sans.
Normally the warning will be just ignored, but if executed in the context of a cronjob it will cause the job to exit.
In your code the Font is defined in Line 308:

ylabel_kws = dict(
        fontsize=32,
        color=kwargs.get('fillcolor', 'whitesmoke'),
        fontweight='bold',
        fontname='Arial',
        ha='center')

Could this be changed to fontfamily = 'serif' or to a parameter?

Since I don't have admin rights on this machine there is no way for me to just install the missing Font.