Grain size distribution histogram in relative unit (Y-axis) vs grain size in micron (X)

[Dan]

Hi
 
I want to plot Grain size distribution histogram in relative unit (Y-axis) or  vs grain size in micron (X) but i am gating grain size distribution histogram on x axis grain size showing from 1 to 15 number as shown in the documentation. this 1 to 15 number can we convert in micron so i can gate the required graph.
or
can we get grain size(area,perimeter,diameter) directly in micron or how convert into micron.

I tried by using conversion factor (pixel to mm) but this is not matching with data. A required graph included in the attached .jpg file  Any guidance would be appreciated!  :)
thanks.