Paste CSV with time or category labels in the first column and one column per series, choose stacked or overlapping, set the theme and labels, then download SVG or PNG.
Stack them when the series are parts of a whole, such as revenue by product line, because the top edge then shows the total. Overlap them with transparency when the series are independent measures you want to compare directly, such as two regions' sales.
In overlapping mode yes: the axis extends below zero and a stronger zero line is drawn so you can see which points fall under it. Stacked mode is designed for non-negative parts of a whole, so mixing signs there produces a chart that is hard to read honestly.
Yes. When the x-axis becomes crowded the tool rotates the labels and then skips them at a regular interval so they never overlap, while every data point is still drawn. Very long datasets are capped for rendering speed with a clear notice.