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## Plotting time derivatives

Plotting time derivatives is much like plotting the difference between the current time state and a fixed time state except that instead of being fixed, the second time state being compared is free to move relative to the current time state. To plot a simple time derivative such as the current time state minus the last time state, create an expression similar to the following expression:

<mesh/ireg> - conn_cmfe(</usr/local/visit/data/dbA00.pdb[-1]id:mesh/ireg>, mesh)

The important piece of the above expression is its use of "[-1]id" to specify a time state delta of -1, which means add -1 to the current time state to get the time state whose data will be used in the conn_cmfe calculation. You could provide different values for the time state in the [] operator. Substituting a value of 3, for example, would make the conn_cmfe expression consider the data for 3 time states beyond the current time state. If you use a time state delta, which always uses the "d" suffix, the time state being considered is always relative to the current time state. This means that as you change time states for the active database using the time slider, the plots that use the conn_cmfe expression will update properly. figure shows an example plot of a time derivative.

Figure 19

Figure 20