Question

I need help in plotting date vs number graph using CorePlot. I have already checked out DatePlot. But my requirement is bit different which is as follows.

I have an array of objects where each object has got a NSDate and a Double number.
For ex:
Array of 5 objects: (NSDate in format yyyy-mm-dd)

• Object1 – 2012-05-01 – 10.34
• Object2 – 2012-05-02 – 10.56
• Object3 – 2012-05-03 – 10.12
• Object4 – 2012-05-04 – 10.78
• Object5 – 2012-05-05 – 10.65

This data comes from a service and would differ every time.

Please advise.

Answer 1

I used a CPTScatterPlot to display a graph of time series data like yours.

You need to create a data source class which will be queried by core plot when it is drawing the graph. My data source object contains an NSArray of objects with two attributes: observationDate and observationValue. The class has to implement the CPTPlotDataSource protocol. These are the protocol methods that I implemented:

#pragma mark- CPPlotDataSource protocol methods
- (NSUInteger)numberOfRecordsForPlot:(CPTPlot *)plot
{
   // return the number of objects in the time series
   return [self.timeSeries count];
}

-(NSNumber *)numberForPlot:(CPTPlot *)plot 
                     field:(NSUInteger)fieldEnum 
               recordIndex:(NSUInteger)index 
{
  NSNumber * result = [[NSNumber alloc] init];
  // This method returns x and y values.  Check which is being requested here.
  if (fieldEnum == CPTScatterPlotFieldX)
  { 
    // x axis - return observation date converted to UNIX TS as NSNumber
    NSDate * observationDate = [[self.timeSeries objectAtIndex:index] observationDate];
    NSTimeInterval secondsSince1970 = [observationDate timeIntervalSince1970];
    result = [NSNumber numberWithDouble:secondsSince1970]; 
  }
  else
  { 
    // y axis - return the observation value
    result = [[self.timeSeries objectAtIndex:index] observationValue];
  }
  return result;
}

Note that I am converting the date to a double – dates cannot be plotted directly. I implement other methods on the class to return values such as the start and end dates of the time series and the min/max values – these are useful when configuring the PlotSpace of your graph.

Once you have initialised your data source you then assign it to the dataSource property of your CPTScatterPlot:

...
CPTXYGraph * myGraph = [[CPTXYGraph alloc] initWithFrame:self.bounds];

// define your plot space here (xRange, yRange etc.)
...

CPTScatterPlot * myPlot = [[CPTScatterPlot alloc] initWithFrame:graph.defaultPlotSpace.accessibilityFrame];

// graphDataSource is your data source class
myPlot.dataSource = graphDataSource;
[myGraph addPlot:myPlot];
...

Have a look at the CPTTestApp in the core plot download for details of configuring your graph and plotspace. If you need any more details please ask. Good luck!