truncate oldest data instead of most recent data

app/activities/TransactionEditor.py

@@ -147,7 +147,11 @@ class TransactionEditor(FocusWidget):
         self.rating.update_rating(_avg, _min, _max, unit, price=price, quantity=quantity)
 
         # after truncating, need to recalculate avg(median), min, max
-        df = df.sort_values('ts_raw', ascending=True, ignore_index=True).truncate(after=self.graph._canvas_width-1)
+        df = df.sort_values(
+            'ts_raw', ascending=True, ignore_index=True
+        ).truncate(
+            before=max(0, len(df.index)-self.graph._canvas_width)
+        )
         data = df[['$/unit','quantity']].apply(
             lambda x: (float(x['$/unit']), float(x['quantity'])),
             axis=1, result_type='broadcast'