Where the start point of a graph is not the ‘true origin’ of the data (0,0), but starts at some other value on one or both axes. The baseline is the y coordinate of the false origin and the basepoint is the x coordinate. There are often good reasons for choosing a false origin, false baseline or false basepoint in order to explore or demonstrate a small effect, but without care this can also lead to misinterpretation or even be used for deliberate misinformation.
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