For the x-axis (time scale), if I rescale it when the application is running (for example I want 15000 points per graph instead of 10000), I get this error message :
If you add data to the graph in vectors, obviously both vectors have to be the same size. If you specify 10000 for x and 15000 for y, then what is the graph supposed to do for the extra 5000 points missing from the x vector? Make something up?

plot->graph(i)->setData(x, matrix[i]); // the graphs receives their new data. From the documentation, this should work fine...
I think you misunderstand the documentation. Where do you resize "x" to be the same size as "matrix[i]"? There has to be a one-to-one correspondence between the x and y vectors - every x value is matched to the corresponding y value (x[i], y[i]) when creating the point or line segment on the graph. So if you add 5000 more y points to the graph, you have to also specify the extra 5000 matching x coordinates.