PLplot looks extremely promising. I am interested in using it as wxPlot. Unfortunately, it lacks one of the three basic chart types that are on my shopping list for a good charting library: financial charts - especially candlestick ones (the other two being pie and line). Like scientific plotting, the financial world has special needs too.
Ideally, good financial charts should include the following features:
1. Choice of chart type: line, OHLC, or candlestick.
2. A seperate volume chart below candlestick charts has been traditional since medieval Japan, when candlestick charts were invented. Even so, this bottom chart should be optional.
3. The candlesticks themselves should have adjustable widths and adjustable wick widths. Ordinarily, you use one color for up days, and another for down days; but for unknown reasons, the SharpCharts at stockcharts.com use three colors. White candlesticks are often drawn as black outlines with black wicks, but the wick does not continue through the body of the candle.
4. The ability to indicate buys and sells, often via green and red triangles that point up and down, and which might appear either above or below the candlesticks or data points.
5. Callout annotations: boxes with arrows pointing to individual candles allow someone to point out individual features of the data.
6. Bollinger bands are very popular, but might be implemented with a simple line chart which overlays the candlestick chart. Still, any indicators that could be included would save programmers a lot of time recreating them: e.g. stochastic oscillators, RSI, moving averages, etc. Moving averages and Bollinger bands usually reside on the candlestick chart (although the former is more often used with OHLC and line charts), whereas the other indicators typically occur in additional windows below that. Although financial charts can seem daunting, the matter can be simplified at first by supporting indicators by simply allowing additional curves to be added either to the main chart, or in a separate chart below the main one (which, of course, uses an identical date axis, which may or may not be indicated for every chart which is part of the plot).
Note that candlestick and OHLC charts do not typically need legends, as they virtually all have only one data series. When more than one security (i.e. stock) is charted, line charts are usually used.
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