I don't understand how to get the temperature from this graph?

[MissP.25_5]

This is an experiment on plasma control. A graph of current against voltage is drawn on a semi log paper. From this graph, we can get the electron temperature (measured in eV). The method is shown in this picture, but I don't understand it at all. Can someone please explain it to me?

 

[CWatters]

Not my field but looks like they picked two points separated by a factor of 270uA/100uA = 2.7 = e on the vertical scale for convenience... ln(e)=1

Something like that anyway.