Handling short positions in Holdings pie chart

[pfalcon]

PP doesn't properly support short transactions and positions (#353), and it would be nice to start addressing them piece by piece. We can start with something relatively "trivial" like Holdings chart (Reports -> Statement of Assets -> Holdings).

Current behavior: PP shows an icon at the top right corner of the chart with warning: "The holdings chart is not correct because there are positions that have a negative valuation. The percentages shown are relative to the total value of the portfolio {0}. The percentages are not relative to the total value of the displayed positions." The short (negative) positions are then not shown on the chart. The percentages shown for long positions may be surprising, e.g. it's possible to see >100% shown for a position.

To understand how to show short position on the chart, it would help to understand and make explicit what the holdings pie chart is intended/expected to show at all. It's fair to say that it's supposed to show relative magnitude of different positions. E.g., if there's Pos1 with value 200, and Pos2 with value 100, then there should be 2 sectors, first being 2/3 of chart, and another 1/3. That shows that magnitude of Pos1 is twice that of Pos2, while they add up to 100% (66.7% + 33.3%).

For short positions, the chart should show the same - relative magnitude. So, if there's Pos3 with value 200 and Pos4 with value -100, then the chart should still show that magnitude of Pos3 is twice that of Pos4, while they still take the whole chart (100%). However, label-wise, the short position should be marked as such, and obvious way would be to show percentage as negative, thus there would be 66.7% and -33.3% labels. Then following will be the invariant: the sum of absolute values of percentages should be 100%. Hopefully, that's still easy enough for humans to comprehend. Would it be useful to add up the signed percentages? Yes, and that would show whether, and how much, the portfolio is net long/short. With the earlier example, the portfolio is ~33.4% net long. With positions of 100 and -100, it would be 50% and -50%, and portfolio would be neutral (another interpretation is that there's exactly enough long assets to cover the short liabilities). With positions 100 and 0, the portfolio is 100% net long. With positions 0 and -100, the portfolio is -100% net short. As can be seen, "net short/long" value would be ranged -100%..100%, while absolute values will always add up to 100%.

The idea is thus to add methods which will return/use magnitude (absolute) value of a portfolio instead of just portfolio balance. Holdings chart should use that magnitude value as a total, and adhoc filtering of negative-valued positions should be removed (abs value should be used where needed, e.g. for pie sector value). The warning message should be updated to convey ideas above, and ideally to should net long/short values (which won't be immediately visible to a user otherwise).

I'm prototyping this implementation.

[pfalcon]

Would it be useful to add up the signed percentages? Yes, and that would show whether, and how much, the portfolio is net long/short.

Ok, that again would give us some funky indicator in the range of -100%...100%. The only benefit of it is if it goes negative, it's clear that the portfolio is net short. However, more natural range is 0%...100%, which would show which share of the portfolio is long. It still would be easily to grasp in from the chat itself: by definition, it's 100% minus sum of all (absolute values of) negative percentages. e.g, if there's 1500 and -500, 75% and -25% shown, 75% of portfolio is long, as expected.