Add a tooltip showing current histogram state on bar histogram.

src/bar_histogram.c

@@ -62,6 +62,10 @@ static void bar_pane_histogram_update(PaneHistogramData *phd)
 	phd->pixbuf = gdk_pixbuf_new(GDK_COLORSPACE_RGB, FALSE, 8, phd->histogram_width, phd->histogram_height);
 	gdk_pixbuf_fill(phd->pixbuf, 0xffffffff);
 	histogram_draw(phd->histogram, histmap, phd->pixbuf, 0, 0, phd->histogram_width, phd->histogram_height);
+
+#if GTK_CHECK_VERSION(2,12,0)
+	gtk_widget_set_tooltip_text(phd->drawing_area, histogram_label(phd->histogram));
+#endif
 }
 
 
@@ -328,6 +332,9 @@ GtkWidget *bar_pane_histogram_new(const gchar *title, gint height, gint expanded
 
 	g_signal_connect(G_OBJECT(phd->drawing_area), "button_press_event", G_CALLBACK(bar_pane_histogram_press_cb), phd);
 
+#if GTK_CHECK_VERSION(2,12,0)
+	gtk_widget_set_tooltip_text(phd->drawing_area, histogram_label(phd->histogram));
+#endif
 	gtk_widget_show(phd->widget);
 
 	file_data_register_notify_func(bar_pane_histogram_notify_cb, phd, NOTIFY_PRIORITY_LOW);