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POUpdating X axis on scrolling real-time CorePlot Scatterplot
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copied!<p>I'm trying to implement a real-time scatter plot using CorePlot 1.0 on an iPad with iOS 5.1. Things are working well with a couple of issues and one major exception - axis redraw.</p> <p>When enough data is collected, I adjust the range in the plotSpace thus:</p> <pre><code>CPTXYPlotSpace *plotSpace = (CPTXYPlotSpace *)self.graph.defaultPlotSpace; plotSpace.xRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromFloat(self.graphMinX) length:CPTDecimalFromFloat(self.graphRangeX)]; </code></pre> <p>When I do this, the plots on the graph adjust as if the axis had changed, <em>but the axis doesn't adjust</em> - so the plots of the data display correctly against an incorrect axis. The axis will correctly update 5 seconds after I stop the data source.</p> <p>I have reviewed the code in <a href="http://code.google.com/p/core-plot/source/browse/examples/CorePlotGallery/src/plots/RealTimePlot.m" rel="nofollow">RealTimePlot</a> (RTP) from the CorePlot iOS Plot Gallery and I can't find any <em>significant</em> difference (though one surely exists).</p> <p>One difference between my code and RTP:</p> <p>I capture the new data in a background GCD queue, which is then "distributed" by attaching it to a custom notification in <code>[NSNotificationCenter defaultCenter]</code></p> <p><strong>Update:</strong> A simplified view of the architecture hierarchy is something like this:</p> <ul> <li>SplitViewController</li> <li><ul> <li>DetailViewController</li> </ul></li> <li><ul> <li><ul> <li><code>TreatmentGraph</code> object (managing the <code>CPTXYGraph</code>)</li> </ul></li> </ul></li> <li><ul> <li><ul> <li><ul> <li>[Collection of] <code>TreatmentChannel</code> objects (each managing a <code>CPTXYPlot</code>)</li> </ul></li> </ul></li> </ul></li> </ul> <p>The <code>DetailViewController</code> has an observer for the data notification which looks like this:</p> <pre><code>- (void)dataArrived:(NSNotification *)notification { FVMonitoredSignal *sig = [notification object]; NSValue *currValue = [sig.dataPoints lastObject]; CGPoint point = [currValue CGPointValue]; [self.treatmentGraph addPoint:point toChannelWithIdentifier:sig.signalName]; dispatch_async(dispatch_get_main_queue(), ^{ [self.graphHostingView.hostedGraph reloadData]; }); return; } </code></pre> <p>(Note that I force a reload of the data using a GCD post to the UI queue - the example in RTP did not seem to require this) This is a red flag, but of what?</p> <p>Within the <code>TreatmentGraph</code> we check whether a X Axis adjustment is needed and the data is dispatched to the appropriate <code>TreatmentChannel</code>.</p> <pre><code>- (void)addPoint:(CGPoint)point toChannelWithIdentifier:(NSString *)identifier { // Check for a graph shift if (point.x >= (self.graphMinX + self.graphRangeX)) { [self shiftGraphX]; } FVTreatmentChannel *channel = [self.channels objectForKey:identifier]; [channel addPoint:point]; return; } - (void)shiftGraphX { CPTXYPlotSpace *plotSpace = (CPTXYPlotSpace *)self.graph.defaultPlotSpace; plotSpace.xRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromFloat(self.graphMinX) length:CPTDecimalFromFloat(self.graphRangeX)]; } </code></pre> <p>My guess is the axis isn't updated until the main queue is idle, but since I'm already forcing a reload when new data arrives, I'm perplexed why the axis redraw doesn't happen then.</p> <p>The <code>TreatmentChannel</code> accepts the new data like this:</p> <pre><code>- (void)addPoint:(CGPoint)point { [self.plotData addObject:[NSValue valueWithCGPoint:point]]; // cache it [self.plot insertDataAtIndex:self.plotData.count-1 numberOfRecords:1]; [self.plot reloadData]; } </code></pre> <p>Note that I'm using <code>-insertDataAtIndex:numberOfRecords:</code> to add just the new data and calling <code>-reloadData</code> specifically on the <code>CPTXYPlot</code>. This is not causing a display update - it's not until the <code>-reloadData</code> is called in the data notification handler in the <code>DetailViewController</code> that I get a display update.</p> <p>Questions:</p> <ol> <li>What can I do to cause my axis to update in a more timely manner?</li> <li>Any clues why I don't get plots on my graph unless I force a reload when data arrives?</li> </ol> <p>Item 1 was addressed by making sure any updates to the axis and/or plotspace were wrapped to put them back on the GCD main queue.</p> <p>Item 2 was addressed by wrapping the calls to <code>-insertDataAtIndex:numberOfRecords:</code> allowed the removal of many of the <code>-reloadData</code> calls that were bothering me.</p> <p><strong>Moral of the story</strong>: consider interaction with CorePlot equivalent to UIKit calls - in terms of making sure they all happen on the main queue.</p>

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