What you see in a graph is dictated not only by the data, but the extents over which you view that data. Simply put, a graph's extents define the minimum and maximum values shown in each of the axes.
By default, a graph's extents are automatically calculated based on the data contained in the measurement so that relevant data is always made visible. When changing the measurement records you have selected, FuzzMeasure will re-calculate the extents so that the data is always made visible. Each graph has its own logic for constraining the automatic calculation of extents, but the fact that extents are calculated automatically is universal to all graphs.
When you modify the extents of a graph, FuzzMeasure will lock the extents and suppress its automatic extents calculation so that things stay where you left them. This allows you to create a viewport in your graph that is consistent across selections, so that you could produce separate graphs of measurements for comparison purposes, or to match a specific range.