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Interpolates between data points and reports an interpolated amplitude at a userspecified abscissa. The method of interpolation is dependent on the type of scaling in use. On a linear scale, linear interpolation is used. On a logarithmic scale, linear interpolation is performed on the logarithms of the data points, and the inverse logarithm of the interpolated value is reported.
Note that you may get wildly different answers between one scaling type and another, particularly if the data points are widely spaced. An example of this apparent discrepancy is illustrated by the following two figures.

Related macro commands 
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https://www.dplot.com/help/index.htm?helpid_interpolate.htm