In the image shown below, the waveform is cuttoff at the left edge, as shown by the red arrow.
Consider that the amount of memory in you computer is finite, so it follows that there must be some maximum amount of data points that can be displayed in the buffer before a limit is reached. This number is about 200,000 data points. After this limit is reached, new data overwrites the oldest data such that only the newest data is retained. (FIFO.)
So what control do you have over this situation?
You can increase the effective size of the buffer for the things that you care about by disabling storage of things that you don't care about. This is done by clicking on the "set" button seen at the bottom left of the picture (above.) Click on the "set" button and the following dialog box opens.
These options are available because all of these take a lot of storage space, because these signals transition quite often (data is stored only on transitions)