Flipping a line graph

I have created a line graph from data supplied by a database, and it looks fine, except it is upside down, ie, the zero value is at the top and the highest value is at the bottom.

Can someone point me in the right direction to correct this?

Y scale goes from 0 at the top to lets say 500 at the bottom.

So a data point of (10,20) would be at (10, 500-20) or (10,480)

A data point of (20,400) would be at (20, 500-400) or (20,100)

[quote=286715:@Markus Winter]Y scale goes from 0 at the top to lets say 500 at the bottom.

So a data point of (10,20) would be at (10, 500-20) or (10,480)

A data point of (20,400) would be at (20, 500-400) or (20,100)[/quote]
Thanks Markus

I think that is enough to understand how it works. The data comes from a database and would be different for each graph, so I essentially have the find the MIN and MAX of each database data set, and then use what you gave me to give me the result I want.