For everyday use of JpGraph understanding of the alignment of text strings in not necessary. However, if you like to add arbitrary strings to the graph (with Graph::AddText()) or when working directly on a canvas it will help understand this.
Text is added to a graph with the creation of a Text() object. And the alignment is specified with Text::Align() Text alignment might actually be a misguiding name. What you specify is rather the anchor point for the text, i.e. when you specify that a text should be positioned at position (x,y) how is that coordinate to be interpretated.
The image below shows a text string aligned in the 9 possible combinations. In the image the red crosses indicate what coordinate that text string was positioned at. The alignment used for each of the cases is shown below.