Equal Intervals command (Edit menu) JR Viewer    Generates a new curve with a constant spacing on the abscissa (X axis). This command is useful for generating evenly spaced data points prior to performing some function that requires this characteristic (FFT for example). It may also be used to generate a smooth curve through widely spaced data points.

This command uses one of four methods for interpolating amplitudes between existing data points. The Linear, Quadratic, and Cubic interpolation methods use, respectively, 1st, 2nd, and 3rd order polynomials.

If the input curve is not sorted in increasing X order, a sorted copy of the input is generated and operated on. Similarly, points with duplicate X values are removed from the input, with the Y value at the point set to the average of all points sharing that same X. Neither of these operations alter the input data. Linear example:  Quadratic example:  Cubic example: Polynomial interpolation uses a finite number (2,3, or 4) of "nearest neighbor" data points to give interpolated values of the input record. In general, the interpolated values do not have continuous first or higher derivatives. That happens because, as X crosses the tabulated input values, the interpolation scheme switches which input data points are used to derive the polynomial interpolating function. If continuous first and second derivatives are important the Cubic Spline method should be used. The cubic spline implemented by DPlot sets the second derivatives at each end of the input record to zero. Cubic Spline example: Select the Replace existing curve checkbox to replace the original curve with the evenly spaced data. Leave the box unchecked to preserve the original data and produce a new curve.

If Use start, end X values is unchecked, the X values for the new curve will range between the minimum and maximum X values for each selected curve. If Use start, end X values is checked, then the specified X limits will be used for all selected curves. Related macro commands

EditEqualInts