The following set of X and Y values plot as a stairstep plot, using an
XY (Scattter) plot, with lines and no markers. In my version of XL
(2002) this is the bottom right icon that shows when you select XY
(Scatter) plot.

X Y
1 1
2 1
2 2
5 2
5 3
6 3

Note the ploy of repeating an X value with the Y value one more than
the previous value (to give you the step-up of one), then repeating the
Y value for the the next X, to give you the horizontal line.

Is this what you want?

HTH