JFreechart Realtime Combined Plot - Rendering the previous value for a step subchart if no data point has been received

Question

I have a combined plot in JFreechart consisting of 4 timeseries charts with a common time domain axis.

The data for the subplots arrives at different rates. For example I may receive a price change event every couple of miliseconds and a position change every minute. I'm currently rendering these series with the XYStepRenderer to produce a step charts.

I'd like to be able to do something like this:

• A Price point is received at time T and is plotted on chart
• No data has been received at time T for the position chart and so we assume that the value has not changed and the previous position value is rendered - i.e. extending the step line horizontally
• At time T+1 a position point is received and is plotted changing the step chart
• At time T+1 no price point has been received and so the previous price is plotted

Is there any easy way to do this in JFreechart? Currently I have a working implementation but this does not handle rendering the assumed values.

My initial thoughts have been around changing the data model so that I can quickly find the previous value and to change the calls to tell each subplot to draw when an event is received for any plot.

Answer 1

I think you'll need to buffer the incoming changes and apply a ruleset for forwarding changes onto JFreeChart (which is not really recommended for real-time work btw., but I've found that for low frequency updates it works well).

I'm guessing you have a dataset for the prices and a dataset for the positions. Do your position and price updates come in with a timestamp ? If not, you'll have the problem of receiving these sequentially and trying to tie them up (perhaps updates arriving within a certain timewindow map to the same time instant?). If they do have timestamps, then you can derive a new position (same as the old position) when the price comes in, and then replace that if need be.

I would decouple the chart updating from the position/price updates whatever happens, just in case you get swamped with position/price changes. You don't want that to translate into multiple chart updates that you're going to have to throw away immediately.