Charts' axis definition

Hello,

I am trying to join 2 kind of graph in one : a heat map and an XY chart.

I need that to see where a value is located on a heat map.

I don't really know what are the best practices for that but for the moment I first build a heat map that I write as a .png image file ("PNG chart and Canvas writer" component) and then I set the background of the XY chart with this PNG.

I have however already a problem:

My heat map data are stored in a file in this way:

X          Y     val

200     -2     val1

200     -1     val2

...

200     7     val10

201     -2     val11

...



And the Y axis goes from 7 to -2 (bottom to top), but I'd like the contrary and I don't know how to change that.

I'll be glad to know how to fix that and/or if this is a good way to do what I want to do.