By default all, statements contain a timestamp that is based on UTC. Setting a time zone in a visualisation will adjust the times displayed in your visualisation based on the time zone you've selected. This is done by subtracting or adding hours from the statement timestamp and then using that adjusted timestamp to plot the visualisation. This can cause a statement to appear on a different day in the visualisation if adjusting the time zone causes the timestamp to advance to the next day or back track into the previous day.
To adjust the time zone of a visualisation when creating or editing on it, go to OPTIONS and select a time zone from the Timezone drop down menu. Once you've done that, click Refresh Chart to see the updated visualisation.