1. A video test apparatus for graphically plotting a value of a video parameter, comprising: a source of time sampled input data values representing a video parameter at incremental sample times; a digital filter operable to produce from each of the input data values a set of plural output data values, wherein the output values comprise interpolated additional values of the parameter between the sample times; wherein the digital filter has plural sets of coefficients applied to the input data values through at least one computation stage, wherein each coefficient in the plural sets produces from a corresponding input data value a product, the products for said plural coefficients and sets contributing to the output values including said interpolated additional values; wherein the coefficients of the digital filter are constant during at least a phase of operation of the interpolation apparatus during which plural input values are processed; further comprising a delay control coupled to the digital filter, the delay control varying a delay input to the digital filter over time while plotting said value of the video parameter, so as to generate the additional values of the parameter between the sample times; and, a display generator operable to plot a graph of the output data values versus an axis representing time, including plotting said additional values of the parameter between the sampled values to produce a more continuous line.