fix an ancient bug in retrieving the data with the cursor

from a log y graph: use y-offset instead of x-offset

src/frontend/plotting/x11.c

@@ -668,7 +668,7 @@ X_ScreentoData(GRAPH *graph, int x, int y, double *fx, double *fy)
     {
         lmin = log10(graph->datawindow.ymin);
         lmax = log10(graph->datawindow.ymax);
-        *fy = exp(((graph->absolute.height - y - graph->viewportxoff)
+        *fy = exp(((graph->absolute.height - y - graph->viewportyoff)
                    * (lmax - lmin) / graph->viewport.height + lmin)
                   * M_LN10);
     } else {

src/frontend/wdisp/windisp.c

@@ -1017,7 +1017,7 @@ static void WIN_ScreentoData(GRAPH *graph, int x, int y, double *fx, double *fy)
     {
         lmin = log10(graph->datawindow.ymin);
         lmax = log10(graph->datawindow.ymax);
-        *fy = exp(((graph->absolute.height - y - graph->viewportxoff) *
+        *fy = exp(((graph->absolute.height - y - graph->viewportyoff) *
                    (lmax - lmin) / graph->viewport.height + lmin) * M_LN10);
     } else {
         *fy = ((graph->absolute.height - y) - graph->viewportyoff) *