diff --git a/roar-python/libgui/GuiCanvasWxBackend.py b/roar-python/libgui/GuiCanvasWxBackend.py
index 31b5eda..54a3cd8 100644
--- a/roar-python/libgui/GuiCanvasWxBackend.py
+++ b/roar-python/libgui/GuiCanvasWxBackend.py
@@ -213,7 +213,10 @@ class GuiCanvasWxBackend():
             dc.SetFont(self._font); self.cellSize = dc.GetTextExtent("_");
             dc.Destroy()
         else:
-            self._font = wx.Font(self.cellSize[0] + 1, wx.FONTFAMILY_TELETYPE, wx.FONTSTYLE_NORMAL, wx.FONTWEIGHT_NORMAL)
+            self._font = wx.Font(self.fontSize, wx.FONTFAMILY_TELETYPE, wx.FONTSTYLE_NORMAL, wx.FONTWEIGHT_NORMAL)
+            dc = wx.MemoryDC()
+            dc.SetFont(self._font); self.cellSize = dc.GetTextExtent("_");
+            dc.Destroy()
         winSize = [a * b for a, b in zip(canvasSize, self.cellSize)]
         if self.canvasBitmap == None:
             self.canvasBitmap = wx.Bitmap(winSize)