Make a PNG-image column with python

[l.thomas]

@imagejan

Hm, this rather suggests that the string buffer is not UTF8-encoded.

By the way, I just realized that the XML string for the SVG figure differs between Python 2/Matplotlib 2.2.3 and Python3/Matplotlib 3.0.3.
In Python3, the string was encoded as byte so the command has to be modified with .decode("utf-8")

plt.savefig(buffer, format='svg')
output_table['plot']=[buffer.getvalue().decode("utf-8")]

I expect something similar for PNG with a different argument for decode but I could not find out.