The re-painting due to mouse-click under MS Windows is caused by line 561

of windisp.c. Upon left mouse button up the plot recangle is invalidated,
and a WM_PAINT message is generated (same on line 614 for right mouse
button up). If not re-pained, artifacts may be left on the canvas after
collecting coordinate data.

Shifting the command InvalidateRect to a place where it becomes active
only after the mouse has been moved, does the trick. Simple clicking
(right or left) will not cause a re-paint any more.

src/frontend/wdisp/windisp.c

@@ -558,7 +558,6 @@ LRESULT CALLBACK PlotWindowProc(HWND hwnd, UINT uMsg,
     case WM_LBUTTONUP:
     {
         GRAPH *gr = pGraph(hwnd);
-        InvalidateRect (hwnd, NULL, TRUE);
         xe = LOWORD (lParam);
         ye = HIWORD (lParam);
         WIN_ScreentoData(gr, xe, ye, &fxe, &fye);
@@ -578,6 +577,8 @@ LRESULT CALLBACK PlotWindowProc(HWND hwnd, UINT uMsg,
         }
         else  {
             /* need to print info about two points */
+            /* trigger re-plotting the graph to get rid of the coordinate rectangle */
+            InvalidateRect (hwnd, NULL, TRUE);
             fprintf(stdout, "\nx0 = %g, y0 = %g    x1 = %g, y1 = %g\n",
                     fx0, fy0, fxe, fye);
             fprintf(stdout, "dx = %g, dy = %g\n", fxe-fx0, fye - fy0);
@@ -611,7 +612,6 @@ LRESULT CALLBACK PlotWindowProc(HWND hwnd, UINT uMsg,
     case WM_RBUTTONUP:
     {
         GRAPH *gr = pGraph(hwnd);
-        InvalidateRect (hwnd, NULL, TRUE);
         xe = LOWORD (lParam);
         ye = HIWORD (lParam);
         /* do nothing if mouse curser is not moved in both x and y */
@@ -619,6 +619,9 @@ LRESULT CALLBACK PlotWindowProc(HWND hwnd, UINT uMsg,
             SetFocus(swString);
             goto WIN_DEFAULT;
         }
+        /* trigger re-plotting the graph to get rid of the coordinate rectangle */
+        InvalidateRect(hwnd, NULL, TRUE);
+
         WIN_ScreentoData(gr, xe, ye, &fxe, &fye);
 
         strncpy(buf2, gr->plotname, sizeof(buf2));