Reporting services stepped line chart problem

We have created a report with a stepped line chart to show the behavior of multiple binary signals. These should obviously be presented as either 1 or 0 depending their value.

A report containing the signal behavior is generated daily and can contain, depending on the presented signals, up to 1000 value changes per day.

Here comes the problem:

When for example a signal called start is presented on its own the stepped line draws just fine. There are only 10 or so value changes of this signal and all 1's and 0's are shown correctly.

However when we want to present two series, the above mentioned signal and another signal with 800 or so value changes, the line of the start signal gets messed up.

What was a nice line before, is now shown as a square like figure showing both a 0 and a 1 line when the value is actually 1 and the other way around. At the moments an actual change in value occurred there is a vertical line drawn so the change moments are still recognized. The result is chart showing boxes instead of lines.

Has anyone ever experienced an inconsistent representation of a record set like this?

Is it possible I'm presenting too much data? Or are the x-axis values (timestamps) too dense?

I can't seem to find any clarification to this problem.