copying chart from excel into powerpoint

Set all margins to 0, and it will still show the axis elements, though barely. If you still want to rid the remaining white still there, stretch your graph out to the edge. That is, click just inside the white "borders" , grab a "dot" handle on the right or top, and pull it right up to the edge.

That will leave only a tiny white amount, for which I am not aware if you can kill that.

Note that you can have chart titles and legends inside that "gray looking" work area (the plot area), i.e. inside the white "border looking" area. I've heard some think that you couldn't stretch out the plot area because of the chart title, but you can go right to the edge.

You can't cover up the axis values to my knowledge, though I could be wrong.