By default plots are not clipped outside the plot area for performance reasons.
This means that if you have manually specified a scale range and your data is outside that range the plot will be drawn outside the scale and into the margin.
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()