By default, plots automatically recalculate when the dependent material changes, when the plotting properties (like axis range) change, or when the thermodynamic models underlying the material have been altered.
Recalculating plot series can take some time. You can turn off the automatic recalculation for a plot from the Plot menu. When a plot that does not automatically recalculate has been invalidated, it will show that a recalculation is required, rather than showing the plot. You can recalculate a plot manually from the Plot menu.