Fig. 1. Plots of the effective potentials (solid line) and (dashed line). Here h = 5, m0 = 1, g = 0.01 and i = 3.

Figures
The same arguments apply as are given above for tables, i.e. it is preferable to have figures that fit into one column of the text (Fig. 1).
It is most important to leave the right amount of vertical space in the figure declaration to accomodate your figure (i.e. remove the lines and change the space in the example).