A graph label value is changed on scrolling

Scenario:

1. Open "data/samples/FSATA/human_T1.fa".
2. Select "GC Content" graph is the corresponding graphs menu on the toolbar.
3. Zoom in to make the horizontal scrollbar visible in the graphs view.
4. Create a label on the graph: hold Shift and click somewhere on the graph.
   Expected result: there is a label on the graph with some value, e.g "[96899, 40.16]".
5. Scroll the graph a bit, the label should be still visible.
   Current result: the label on the graph has another value, e.g. "[96899, 41]".
   Expected result: the label value is not changed.