decide how to handle logarithmic scales

[ltalirz]

The web plot digitizer automatically handles the (mathematically exact) digitization on logarithmic scales, i.e. when following this approach, there is no reason to ever enter "exponents" into the pressure column (which was its purpose in the original version of the digitizer).
I think it is more natural for users to enter the scale as they see it printed on the figure (e.g. picking points 1e-3, 1e2; select "logarithmic scale" in the webplot digitizer).

However, even though the values entered into the data box are no longer the exponents, it may still make sense to indicate the fact that digitization occurred from a logarithmic pressure scale (e.g. to estimate uncertainties of data points).

We could repurpose the "logarithmic scale" box to simply act as an indication that the data was digitized from a logarithmic plot.

[ltalirz]

Note to self: for isotherms that have the "logarithmic pressure scale" ticked, the plot will by default be shown on the logarithmic scale (if not, it will show as linear; in both cases users will have the ability to switch the scale of the plot as before).