DPlot

I have tried to find out how to get the maximum value of some curves (cartesian coordinate xy graph) for each x value, but I failed. Could you help me?

It really wasn't designed for this, but you can do this with the $YMAX text placeholder. $YMAX(0) gives the maximum Y value for all curves. $YMAX(0,a,b) (where a and b are numbers) gives you the maximum Y value for all curves between a < X < b. So if you make a and b some small delta less than and greater than the X you're interested in, this will do the job... sort of. The returned value may not be exactly at X, but if you make the delta small enough it should be fairly close.