Understanding Core Plot: X Axis Labels Not Displaying Properly
Core Plot is a powerful and versatile plotting framework for iOS, macOS, watchOS, and tvOS. It provides a wide range of features and tools for creating high-quality plots, charts, and graphs. However, when dealing with certain aspects of plot customization, developers may encounter unexpected issues or behaviors.
In this article, we will delve into one such issue: X-axis labels not displaying properly on the first draw of a graph using Core Plot. We will explore the reasons behind this behavior, discuss possible solutions, and provide code examples to illustrate the concepts.
Understanding Core Plot’s Data Sources
Core Plot relies on data sources to retrieve plot-related information, such as data points, colors, and labels. The data source is responsible for providing the necessary data to the plot, which then uses that data to render the graph.
In the provided example, the numberOfRecordsForPlot: method is used as a data source. This method returns the number of records (data points) available for plotting. However, in this case, additional calculations and customizations are performed within this method, which may be contributing to the issue.
Calculating Axis Labels: A Complex Task
Calculating axis labels can be a complex task, especially when dealing with dates or other data types that require formatting and localization. In the provided example, custom tick locations and x-axis labels are calculated using NSDecimalNumber objects and date components.
// Calculate custom tick locations
NSArray *customTickLocations = [NSArray arrayWithObjects:
[NSDecimalNumber numberWithInt:0],
[NSDecimalNumber numberWithInt:1],
[NSDecimalNumber numberWithInt:2],
[NSDecimalNumber numberWithInt:3],
[NSDecimalNumber numberWithInt:4],
nil];
// Calculate x-axis labels
NSMutableArray *xAxisLabels = [[NSMutableArray alloc] init];
for (int i=0; i<self.arrayKeys.count ; i++) {
// ...
NSDate *date = (NSDate *) [self.arrayKeys objectAtIndex:i];
[xAxisLabels addObject:date];
}
The Issue: X-Axis Labels Not Displaying Properly
The issue at hand is that the x-axis labels are not displaying properly on the first draw of the graph. However, when the graph re-draws with new data, the x-axis labels display correctly.
To understand why this is happening, let’s analyze the code snippet:
-(NSUInteger)numberOfRecordsForPlot:(CPTPlot *)plot {
// ...
// Calculate custom tick locations and x-axis labels
NSMutableArray *customLabels = [NSMutableArray arrayWithCapacity:xAxisLabels.count];
for (int i=0 ; i<dictHealth.count ; i++) {
// ...
CPTAxisLabel *newLabel = [[CPTAxisLabel alloc]
initWithText:[NSString stringWithFormat:@"%d-%d-%d\n%d:%d:%d",
dateComponents.year, dateComponents.month, dateComponents.day,
dateComponents.hour, dateComponents.minute, dateComponents.second]
textStyle:textStyle];
// ...
[customLabels addObject:newLabel];
}
axisSet.xAxis.axisLabels = [NSSet setWithArray:customLabels];
// ...
}
In this code snippet, the custom tick locations and x-axis labels are calculated only when the graph re-draws with new data. This is likely why the x-axis labels display correctly on subsequent draws.
The Solution
To resolve the issue of x-axis labels not displaying properly on the first draw of the graph, we need to rethink our approach to calculating axis labels. Here are a few possible solutions:
Pre-calculate and store custom tick locations: Instead of calculating custom tick locations every time
numberOfRecordsForPlot:is called, pre-calculate them in the view controller or data source initialization method. Store these values in an instance variable or property.
// In the view controller or data source initialization method
(void)initializeCustomTickLocations { // Pre-calculate custom tick locations NSArray *customTickLocations = [NSArray arrayWithObjects: [NSDecimalNumber numberWithInt:0], [NSDecimalNumber numberWithInt:1], [NSDecimalNumber numberWithInt:2], [NSDecimalNumber numberWithInt:3], [NSDecimalNumber numberWithInt:4], nil];
// Store the custom tick locations in an instance variable or property _customTickLocations = customTickLocations; }
// In numberOfRecordsForPlot:
-(NSUInteger)numberOfRecordsForPlot:(CPTPlot *)plot { // …
// Use pre-calculated custom tick locations
CPTMutableTextStyle *textStyle = [[CPTMutableTextStyle alloc] init];
textStyle.fontSize = 10.0;
textStyle.color = [CPTColor darkGrayColor];
for (int i=0 ; i<dictHealth.count ; i++) {
NSNumber *tickLocation = [_customTickLocations objectAtIndex:i];
// ...
}
}
2. **Provide default axis labels**: When creating the plot, provide some reasonable default values for axis labels. This can include a range of tick locations and labels that can be used if custom calculations are not available.
```markdown
// In the view controller or data source initialization method
- (void)initializePlot {
// Create a new CPTPlot space
CPTPlotSpace *plotSpace = [[CPTPlotSpace alloc] init];
plotSpace.xRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
plotSpace.yRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
// Create a new CPTXYAxis
CPTXYAxis *xAxis = [[CPTXYAxis alloc] init];
xAxis.range = CPTRangeZeroToSelf();
xAxis.labelPlacement = CPTAxisLabelPlacementOutside;
plotSpace.xAxis = xAxis;
// Create the plot
self.plot = [[CPTPlot alloc] initWithGraphingDiameter:NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf()];
self.plot.space = plotSpace;
self.plot.xAxis.labelPlacement = CPTAxisLabelPlacementOutside;
}
// In numberOfRecordsForPlot:
-(NSUInteger)numberOfRecordsForPlot:(CPTPlot *)plot {
// ...
// Use default axis labels if custom calculations are not available
CPTMutableTextStyle *textStyle = [[CPTMutableTextStyle alloc] init];
textStyle.fontSize = 10.0;
textStyle.color = [CPTColor darkGrayColor];
for (int i=0 ; i<dictHealth.count ; i++) {
NSNumber *tickLocation = [NSDecimalNumber numberWithInt:i];
// ...
}
}
Conclusion
In this article, we explored the issue of x-axis labels not displaying properly on the first draw of a graph using Core Plot. We discussed possible solutions and provided code examples to illustrate these concepts.
By pre-calculating custom tick locations or providing default axis labels, you can resolve this issue and ensure that your plots display correctly.
Additional Considerations
When working with Core Plot, keep the following considerations in mind:
- Data sources: Use data sources to retrieve plot-related information. The data source is responsible for providing the necessary data to the plot.
- Plot customization: Customize your plot by adjusting its properties, such as colors, padding, and axis labels.
- Graph re-draws: When plotting new data, consider how this affects your graph. Re-drawing a graph can have unexpected consequences, especially when it comes to axis labels.
By understanding these concepts and implementing the suggested solutions, you can create high-quality plots that display correctly on all devices.
// Additional code snippet
-(void)plotNeedsUpdate {
// Update plot ranges and axes if necessary
CPTPlotSpace *plotSpace = self.plot.space;
plotSpace.xRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
plotSpace.yRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
// Update axis labels
NSMutableArray *customLabels = [[NSMutableArray alloc] initWithCapacity:10];
for (int i=0; i<dictHealth.count ; i++) {
NSNumber *tickLocation = [_customTickLocations objectAtIndex:i];
NSDateComponents *dateComponents = [self getComponentFromDate:
(NSDate *)[xAxisLabels objectAtIndex:i]];
CPTAxisLabel *newLabel = [[CPTAxisLabel alloc]
initWithText:[NSString stringWithFormat:@"%d-%d-%d\n%d:%d:%d",
dateComponents.year, dateComponents.month, dateComponents.day,
dateComponents.hour, dateComponents.minute, dateComponents.second]
textStyle:textStyle];
newLabel.tickLocation = [tickLocation decimalValue];
newLabel.offset = 7.5;
newLabel.rotation = M_PI/3;
[customLabels addObject:newLabel];
}
self.plot.xAxis.axisLabels = [NSSet setWithArray:customLabels];
}
// Implement -reloadData or -insertDataAtIndex:numberOfRecords:
-(void)plotNeedsUpdate {
// Update plot ranges and axes if necessary
CPTPlotSpace *plotSpace = self.plot.space;
plotSpace.xRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
plotSpace.yRange = NSValue rangeWithDecimalNSRangeValue:CPTRangeZeroToSelf();
// Update axis labels
NSMutableArray *customLabels = [[NSMutableArray alloc] initWithCapacity:10];
for (int i=0; i<dictHealth.count ; i++) {
NSNumber *tickLocation = [_customTickLocations objectAtIndex:i];
NSDateComponents *dateComponents = [self getComponentFromDate:
(NSDate *)[xAxisLabels objectAtIndex:i]];
CPTAxisLabel *newLabel = [[CPTAxisLabel alloc]
initWithText:[NSString stringWithFormat:@"%d-%d-%d\n%d:%d:%d",
dateComponents.year, dateComponents.month, dateComponents.day,
dateComponents.hour, dateComponents.minute, dateComponents.second]
textStyle:textStyle];
newLabel.tickLocation = [tickLocation decimalValue];
newLabel.offset = 7.5;
newLabel.rotation = M_PI/3;
[customLabels addObject:newLabel];
}
self.plot.xAxis.axisLabels = [NSSet setWithArray:customLabels];
}
Last modified on 2025-02-28