I see your point of upward slopes being positive or negative, depending on the displayed parameter. However, as the number of dashboard charts are usually not that many and there is a lot of repeat, this shouldn't be much of a factor.
Anyway, I haven't come across myself, but with a bit creative thinking I believe there are more ways to visualize positive vs negative. Because I don't know the context, nor what software you'r using, here are just a few triggers:-
You could display a pos/neg symbol in the corner of the chart (say an arrow pointing down or up)
You can us a different chart frame colour (red = neg; green = pos) - e.g., with conditional formatting in Excel (or any other attribute for that matter: line colour, line width, ..)
You could think of showing the budget/target as a grey area and with costs (that shd slope downwards) you can show the grey area at the upper side of the chart - "in grey" then consistently means "bad news".
Yet another option is to place the charts with similar characteristics in a separate part of the screen, logically grouped (the charts can have a different colour setting as to strengthen the difference).
Reverse the value axis (i.e., show the lowest values at the top)
You could possibly find many other ones. If you use Excel I can help you further, e.g., this may be a good opportunity to try out my Trendicator Graph (see another post here) - but that requires some space and I don't know whether this is available.
Hope this helps,