As per my understanding, you will not get the type of output you want with bar3. Refer to the documentation of bar3
How it works is you provide a matirx Z, and bar3(Z) will plot a bar graph where each bar corresponds to one element of Z and has the height of that element.
I would suggest you use stem3 instead, this should give you the result you expect
% x 40x1 double
% y 426x1 double
% FlowRate 426x40 double
[X,Y] = meshgrid(x,y);
% X, Y 426x40 double
stem3(X,Y,FlowRate)
Documentation
