3.6.17 Grad sample

Function grad draw gradient lines for matrix. The sample code is:

int sample(mglGraph *gr)
{
  mglData a;  mgls_prepare2d(&a);
  gr->SubPlot(1,1,0,""); gr->Title("Grad plot");
  gr->Box();  gr->Grad(a);  gr->Dens(a,"{u8}w{q8}");
  return 0;
}