Excel plots, like hand-drawn plots, must have titles, labels on the axes, good choice of scale, etc. The results can be disaster, even when the plot looks ok. But modifying an existing plot directly without the wizard is not difficult.
Some examples follow.
To make any changes in a graph, you must first activate the chart. The border of the plot should get black squares at the corners and at the centers of the edges. Often, one inadvertently activates an axis or a curve, if this happens, try again. Go to them menu bar, find the fill button shown by a tipping paint bucket on the upper right corner, click and hold the mouse button, and drag down to get the Fill menu. You can also resize the graph when the plot area is activated by clicking on a corner and moving it.