How to keep the Bar number displayed in chart window unchanged?

 
When a new Bar appear, how to refresh the chart window to keep the total bar number displayed unchanged?
for example if i set the maximum bar number displayed in the chart window to 120, when a new bar appear. it will changed to 121 bars displayed. how can i keep displaying only 120 bars, and eliminate displaying the most left one.
 
When a new Bar appear, how to refresh the chart window to keep the total bar number displayed unchanged?
for example if i set the maximum bar number displayed in the chart window to 120, when a new bar appear. it will changed to 121 bars displayed. how can i keep displaying only 120 bars, and eliminate displaying the most left one.

That looks like a bug.