Charting elevation curves propotionally

When charting elevation cureves in a scatter graph I have a problem with the graph not being propotional.

I would like to enter the hights of the curves and the distance between the curves to see, in a scatter graphs, the sloping - a profile of the land.

But as you change the values the graph looses its 1:1 propotion and the slope gets wrong - I cant figure it out - does anyone have any suggestions?

See attachment - example file.

Thanks!
Patrick