More than one curve or line can be drawn on a single graph for purposes of comparison. The lines can be differentiated by color or broken and dotted lines etc. Common Graphical Errors Missing labels, titles, etc. All graphs must have a title, all axes must be labeled including units. Wrong use of scale.
Most people read increasing scales from left to right and from bottom to top.
Comparative graphs must be plotted on the same axes to facilitate comparisons. Misplaced zero point. Most people assume that the zero point is at the bottom of the graph. This can give a very misleading impression of the amount of change present in a data series. Wrong choice of chart type.