My problem was the black color that covered the series legend.
You might have already figured out the solution. Still I just wanted to let you know that you can change legend item’s font color so that it doesn’t become invisible because of the same background color.
Is it possibile to have a Y interval from +50 to -10 for example (always)
Yes you can set the axis range using minimum and maximum properties of axisX/axisY. Checkout Axis Range section of this tutorial on axis.
Is it possibile to put the legend name inside toolTipContent.
Yes. Here is an example.