Finally I solved the problem. The problem itself was really happening, but the cause was very hidden in my code. After some hour I discover what was wrong.
I will try to explain, so if someone else have a problem like this one, it may be helpful.
Like I said, I am drawing a line and a candlestick graphics. The problem was in the definition of the X axis. I didn't notice before, but in the line series I have used a number for X-axis, and in the candlestick I was using, without perceive, a categorical X axis. So, I think RadChart was re-doing the X-axis to hold that. The solution? Change the X-axis definition in candlestick to a number (through XValue method).
It's a simple problem, but it cost me some time to discover what was wrong.
I hope this may be usefull to someone else.