Multigraph

Sometimes it is helpful to add a horizontal line to a graph that appear at a certain place along the axes but that doesn't correspond to a variable in your data. For example, on a temperature plot you might want to draw a horizontal line that indicates a freezing point. You can do this by creating a separate plot element for the line, and using the constant element inside the plot's verticalaxis element, instead of the variable element. The constant element takes one attribute, value, which is the data value at which the horizontal line should appear. You can specify styling details for the horizontal line, such as what plot style and colors to use, in the normal way for all plots as described in the section called “Plots”, by using the renderer subelement.