To plot any type of graph based on external data you should load an array and give it a label. To load external data use Command -> Load array in Calc window.
Definition for all three type just contains array name, for instance: #a1
Depending on graph type different curves appear.
In the case of array just a broken line joining array points will be plotted.
In the case of 3-spline the graph is a spline of 3rd order, i.e. a smooth function, passing through array points and being a cubic parabola on each segment. For non-ambiguity the second order derivatives are set to zero at the ends of domain.
If a graph of one of these three types is the first plotted in the graph window, the scale is automatically adjusted to show all array points. You can also adjust the scale manually by selecting the graph definition and clicking Command -> Best Fit.