Modifying values on Y axis of a histogram

#1
Hello and sorry in advance for my poor English ;)

I would like to plot a frequency distribution in R. On the X axis are my values (elevation in my case) and on the Y axis the frequency. So far so good. But: I would like to convert this frequency values (ammount of pixels in my case) into hectares. One hectare=13 pixels.

My question: is it possible to simply divide the value of an axis or what would be another way to change them?

Thank you all for your help.