Transparency Using Plot Problems with Alphadata

Basically what I want to do is plot data on one plot, and plot another set of data on a second plot. I want to place both images on top of each other and be able to change the transparency of the top one so the bottom one becomes visible.

I tried using alphadata and what happens is the transparency changes..but I cant see the plot behind it. All I see is white background.. can I not use alphadata on plots?

Additional information:

Imagesc is used to display the data on the plot cannot use imread or imshow