intensity plot - Idl-pvwave

Dear forum users,

I am an absolute beginner with IDL and I have a problem which I can’t solve by myself :-(
I am using IDL Version 7.1 and I try to create an intensity plot.

I have three vectors (1d arrays): x, y, and z. x and y are the coordinates for the z value. x and y are not equidistant (only approximatively).

Until now I used
iimage,z,x,y

which produces with the IDL Gridding Wizzard nice images but not exactly the images I want to have. I do not want that my data are connected by a gridding method like inverse distance and so on.
I just want to plot my data as they can be seen in the IDL Gridding Wizzard before I have to choose a gridding method.

Or with other words:
I want to make a simple x-y plot (as it is done by plot,x,y), and then I want that my z-values are plotted at the corresponding x-y-coordinates with different colours/greyscales. E.g., the lower the z-value is the blacker and the higher the z-value is the whiter the plotted point should be.

Can anybody help me to achieve my aim? I would be very thankful for every help!