Add an image_size parameter to ag_chart_create_svg()

This is required so we can generate a fixed size image, e.g. for the
chart preview.

src/ag-window.c

@@ -532,6 +532,7 @@ ag_window_redraw_chart(AgWindow *window)
             &length,
             FALSE,
             NULL,
+            0,
             &err
         );