A range of Spectrum channel calculations are available in LabChart. Like many other calculations which produce discrete results, these are displayed using step interpolation, i.e. a series of connected horizontal lines. However if you create an Arithmetic channel calculation which combines two Spectrum calculations then the results look quite different, with curves rather than straight lines between the data points.
This behavior is the result of the way the calculation is performed in LabChart, and is because an operation is being done on two channels at a low sample rate (lower than the "tick rate", or rate of the fastest channel). Before performing the mathematic operation, division for example, the two Spectrum channels are interpolated so that they are at the tick rate. Due to the way the software is written, this interpolation for Spectrum calculations is done using linear interpolation (straight line segments, but not necessarily horizontal) rather than step interpolation. This means the division of the two signals is performed using the ratio of two line segments, which creates a curve.
So the results at the actual data points (the vertices of the step-interpolated data) are as expected, but the way the region between those points is displayed may be unexpected. To get a better idea of how this calculation is performed you can open the Scope View, which displays the Spectrum Channel Calculations using the same linear interpolation as the one used by Arithmetic.
NOTE: This issue was corrected in the latest release of LabChart 8, the install for which can be downloaded from HERE.
For further technical assistance with this or any other issue, please contact ADInstruments Technical Support by clicking HERE.