Interpolation in Excel

Hello,

I am plotting two sets of data in one chart.

Excel automatically interpolates the data when creating the graph.

For example, supposing that I have in the x axis days and y axis water levels.

1 set of data is for example:

Data - Water Level
01/01/2000 - 2,7
01/05/2005 - 3,4
01/11/2010 - 2,8

I would like to produce a plot keeping a constant level in the interval between the date, and when it comes the change in water level, having a sudden change in the plot, like a step. But Excel interpolates the different water levels as default.

How can I change that?

While for the second set of data itīs fine the excel interpolation.

I do not want the interpolation just in the first set of data. And I would like to plot the two sets in 1 graph.

Hope itīs clear.
Thanks,
Giovanni