Y Axis Scaling on a Custom Format

I have a Short Call v's Asset Chart (top large chart) using a Custom Format that placed the 0 value at the top of the Y axis but I noticed that the LHS scale is missing incremental values so it jumps from eg 400 to 900 and it also doesn't have the same density of grid lines compared to the large chart directly below it (although) they are set up with the same gridline settings.

Is there anyway to get the top chart to have the same level of gridlines and incremental values (increments of 100, or even 200 would be better than 400 jumps) bearing in mind the top chart has to cover a wider range of values (pls see second image, where the first "SC" column is for the top "Short Call" chart and the second columns for the lower chart) than the lower chart?

Cheers.