By default all plots are clipped outside the plot area. This means that if you manually specify a scale and then try to plot which has values smaller/larger than the scale those values will not show.
The clipping algorithm is "perfect" in the sense that for example line plots where the plot area cuts the line between two data points the line will be drawn up to the edge of the plot area. The algorithm used is O(1) in number of data points.
The disabling/enabling of clipping manually is controlled by Graph::SetClipping()