If I understand your query, the answer is that the bar objects are actually flying in from the very bottom of the window, but there is a WHITE RECTANGLE object covering the bottom part of the screen - from the very bottom of the screen to the bottom of the graph area.
While the bar objects are flying in this bottom part of the screen, they are BEHIND this 'mask' object so you can't see them.
Like many clever effects you'll see in Storyline projects, it's all 'smoke and mirrors'.
Another way a similar effect could be achieved is to have a SCROLLING PANEL about the same size as the chart area with the animated bars INSIDE that. Any animation that happens is relative to the boundary of the scrolling panel rather than the entire screen. So, when the bars fly in, they fly in from the bottom of the scrolling panel, not the screen. It's a bit more complicated, but there's always more than one way to do anything!