Contents Previous Next

6.20 Formatting the axis

To get the exact type of axis you want in your graph there are a number of adjustments available for you. You may change, color, size, position and general apperance.

6.20.1 Standard two axis graphs

Assuming we start with the traditonal two axis graph, one X and one Y axis. You may then change the position of each axis by calling Axis::SetPos($aPosition) You have to remember that you need to specify the position on the other axis. SO you need to specify the world-coordinate for the position. By default the axis are each positoined at the 0-point on the other axis, i.e. the axis will cross at the 0,0 point in the graph.

In additio to the standard positioning you may also use the two special position markers "min" and "max". This will position the axis at the minimum (or maximum) position of the other axis.

For example, to make sure that the X-axis is always at the bottom of the graph (at lowest possible Y-value) you would have to add the line


 

$graph->xaxis->SetPos("min");

To change the color and width of the axis you have to make use of the Axis::SetColor() and Axis::SetWeight() methods.


Invisible axis Even though JpGraph (1.7) doesn't directly support "hidden" axis wher the labels are still drawn it is very easy to achive this effect by setting the colors of the axis to be the same as the background. See the example barintex2.php in the Example directory. To completely hide an axis you can make use of the Hide()

You might also want to add titles to the axis. This is done through the Axis::SetTitle() method. This is actually just a shortcut for accessing the title proeprty direct. Axis::title::Set() which also allow you to set the alignment in one call.

By default the position of the title is to the far right for the X-axis and in the middle (and 90 degrees rotated) for the Y-axis.

You can adjust the position of the title with the help of the second argument to the Axis::SetTitle() method.

The possible positions are "high","middle" and "low" which refers to the scale values on the axis.

One common modification you might want to do to the title is to increase the margin between the axis and the actual title. This is often necessary to do for the Y-axis if the values displayed are large. You may adjust the distance (in pixels) between the axis and the title by using the method Axis::SetTitleMargin()

So for example to increase the margin on the Y-axis you might add the line


 

$graph->yaxis->SetTitleMargin(40);

to your code.

Finally we mention something about the positioning of tick marks and labels on the axis. You have the possibility to choose what side of the axis the tick marks and the labels should be at. For the X-axis this can be specified as either on the the top (inside the plotarera) or at bottom (outside of the plotarea). In the same way you can specify for the Y-axis if the labels ( or ticks) should be on the left or right side.

To adjust the label positioning you have to use the method Axis::SetTitleSide() and to adjust the position of the tick mark you have to use the method SetTickSide()


Note: There is also an alias for this method, SetTickDirection() which is deprecated from version 1.7 but kept for backwards compatibility.
Valid arguments for these methods are For example, the following lines added to a script would change side of the labels and tickmarks for the X-axis.


 

$graph->xaxis->SetLabelPos(SIDE_UP);
$graph->xaxis->SetTickSide(SIDE_DOWN);

This technique is for example used if you position the X-axis at the top of the graph as the following example shows.



Figure 1: Example of both how to adjust the position of the X-axis as well as adjusting the side for the tick and axis title [src]

6.20.2 Scientific style axis

In scientific style plots it is often common to duplicate each axis so that all sides of the graph have a labeled axis. This is of course also fully supported by JpGraph.

Before we continue we show a small example to illustrate this feature



Figure 2: Example of scientific axis [src]

The example above shows the basic configuration. There are now several modifications you may do to these axis like

The style of axis is determined by the method Graph::SetAxisStyle() The available type of axis are

6.20.3 Adjusting the position of the scale labels

How to adjust the actual labels are discussed elsewhere in this manual (see ???,???). Howver we like to mention here that you can adjust the label margin (distance betweeen the axis and the labels) with the method Axis::SetLabelMargin()

to adjust the actual label format (like font, color, angle) you need to access the Axis::SetFont() and the Axis::SetColor() methods. If you investigate the Axis class you will discover more methods to adjust the many aspects of the axis layout.

As a final note we also mention the methods Axis::SetLabelAlign() and Axis::SetLabelAngle() This first method is really only mentioned here for completenesss since it is mostly used for internal purposes. However on som occasion you might want to adjust the alignment of the labels. By default they are centered in respect to the tickmark. By using the method you might override this positioning should you choose to do so.

The second of these methods adjusts the angle of the label in regards to the axis. This is very usefull for X-axis that have long labels.

6.20.4 Formatting the scale labels

[TODO] Graph::SetLabelFormatCallback(); Graph::SetLabelFormat();

6.20.5 Inverting the Y-axis

One good way of illutrsate the usefullness of label callbacks in a slightly different context is to show how we can achieve the effect of an inverted Y-scale.

An inverted Y-scale has the lowest number at the top and the scale values increases downwards.

Even though JpGraph doesn't directly support this feature it is wuite easy to achieve with just a few extra lines of code in your image script.

Before we continue we give an example of what we are referring to.



Figure 3: Inverted Y-axis [src]

Two achieve this effect there are two simple steps to take:

  1. Negate all you Y-value in the data
  2. Create a callback that negates the scale labels so they appear to be positive.
And that's it! We refeer you to the code in the example above for the details.

6.21 Adjusting the autoscaling limits - grace value

By default the autoscaling algorithm tries to make best possible use of screen estate by making the scale as large as possible, i.e. the extreme values (min/max) will be on the top and bottom of the scale if they happen to fall on a scale-tick. So for example doing a simple line plot could look like the plot shown in the below.



Figure 4: A typical graph with autoscaling and grace=0 [src]

However you might sometime want to add some extra to the minimum and maximum values so that there is some "air" in the graph between the end of the scale values and the extreme points in the graphs. This can be done by adding a "grace" percentage to the scale. So for example adding 10% to the y-scale in the image above is done by calling the SetGrace() method on the yscale as in


 

$graph->yaxis->scale->SetGrace(10,10);

These lines add a minimum of 10% to the top and bottom values of the scale. Note that we say "minimum" since depending on the specific tick values choose this might be a little bit more to make the end of the scale fall on an even tick mark.

Adding this line to the previous graph will result in the following example



Figure 5: Adding 10% grace value to top and bottom of the Y-scale [src]

Since we haven't adjusted the positoin of the X-axis it will remain at Y=0 which might not necessary be waht we would like so we therefor also add the line


 

$graph->xaxis->SetPos("min");

So that the X-axis always will remain at the lowest possible Y-value. Doing this will then result in the example below



Figure 6: Using grace but also adjusting the position of the X-axis [src]


Contents Previous Next