Issue with font on Linux

[bartkowiaktomasz]

If you get an error with font on Linux while trying to POST an image, you need to edit the file
CNTK-Python-Web-Service-on-Azure/plot_helpers.py, edit the lines 318, 323 and change:
ImageFont.truetype("arial.ttf", 16)):
to
ImageFont.truetype(available_font, 16)):
as it has been done by @karolzak in:
fixed an issue with font on Linux

[karolzak]

@bartkowiaktomasz thanks for pointing that out!