Core Plot: Custom labels on the x-axis for bar charts

Question

I am using Core Plot bar charts to plot company growth rates. I would like to have the company's stock symbols as the labels on the x-axis centered below their respective bars. Unfortunately I have spent many hours in searching for a way to center the x-labels correctly, but had no success using my code. How can I get the x-axis labels centered correctly?

My chart set-up is as follows:

CPTBarPlot *barPlot     = [CPTBarPlot tubularBarPlotWithColor:[CPTColor blueColor] horizontalBars:NO];

barPlot.baseValue       = CPTDecimalFromInt(0);
barPlot.barOffset       = CPTDecimalFromFloat(0.5f);
barPlot.barWidth        = CPTDecimalFromFloat(0.5f);

double xAxisStart = 0;
double xAxisLength = self.companies.count;

double yAxisStart = 0;
double yAxisLength = 0.5;

CPTXYPlotSpace *plotSpace = (CPTXYPlotSpace *)graph.defaultPlotSpace;
plotSpace.xRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromDouble(xAxisStart) length:CPTDecimalFromDouble(xAxisLength + 1.0)] ;
plotSpace.yRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromDouble(yAxisStart) length:CPTDecimalFromDouble(yAxisLength)] ;

barPlot.plotRange = [CPTPlotRange plotRangeWithLocation:CPTDecimalFromDouble(+0.0) length:CPTDecimalFromDouble(xAxisLength)] ;

In the following code snippet, I tried to use custom labels, but without success as the sample chart below shows.

xAxis.labelingPolicy = CPTAxisLabelingPolicyNone;

NSMutableArray *customLabels = [[NSMutableArray alloc]init];

[self.companies enumerateObjectsUsingBlock:^(IBCompany *company, NSUInteger idx, BOOL *stop) {
    NSString *labelText = company.contrSymbol;
    CPTAxisLabel *label = [[CPTAxisLabel alloc] initWithText:labelText textStyle:xAxis.labelTextStyle];
    label.tickLocation = CPTDecimalFromDouble(idx + 0.5);
    label.offset = xAxis.labelOffset + xAxis.majorTickLength;
    label.rotation    = M_PI * 2;
    [customLabels addObject:label];
    }];
  xAxis.axisLabels = [NSSet setWithArray:customLabels];

barcharts with incorrect label positioning

Please note that my index for the bar charts starts with 1:

-(NSArray *)numbersForPlot:(CPTPlot *)plot field:(NSUInteger)fieldEnum recordIndexRange:(NSRange)indexRange
    {
    if ( [plot.identifier isEqual:plotIdentifier] ) {;

    if ( fieldEnum == CPTScatterPlotFieldX ) {
        NSMutableArray *indexArray = [[NSMutableArray alloc] init];

        [self.companies enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
            [indexArray addObject:[NSDecimalNumber numberWithInt: idx + 1]];
        }];
        return [indexArray copy];
    }
    else if ( fieldEnum == CPTScatterPlotFieldY ) {
        // ..
    }
}
else return nil; // should be considered as ERROR

}

Eric Skroch Eric Skroch · Accepted Answer · 2012-10-07T20:11:49

You should get in the habit of using the field identifiers for the right plot type in your datasource. For bar plots, you should be using CPTBarPlotFieldBarLocation and CPTBarPlotFieldBarTip. In this case there's no difference, but that's not always true with other plot types. For instance, the x and y coordinates are reversed for horizontal bar charts.
The datasource always returns the same dataset. You need to check the indexRange parameter and only return the range requested.
The plot range for the x-axis runs between 0 and 4. The labels are at 0.5, 1.5, and 2.5. The plotRange is what's messing up the bar spacing. Leave it set to the default of nil and you should get the appearance you want. From the plotRange docs:

If a plot range is provided, the bars are spaced evenly throughout the plot range. If plotRange is nil, bar locations are provided by Cocoa bindings or the bar plot datasource. If locations are not provided by either bindings or the datasource, the first bar will be placed at zero (0) and subsequent bars will be at successive positive integer coordinates.

Core Plot: Custom labels on the x-axis for bar charts

1 Answers