Question

I want to visualize some data about cars in a table via Prefuse Java.
Three kinds of important data objects are used:

1. PIT (Point in Time)
2. Value (contains the consumption of gas as a Double)
3. Car (the class that represents a Car)

So first I put them into a table like this:

car1 | pit1 | value11
car1 | pit2 | value12
car1 | pit3 | value13
car2 | pit1 | value21
car2 | pit2 | value22
car2 | pit3 | value23
car3 | pit1 | value31
car3 | pit2 | value32
car3 | pit3 | value33

Using the examples in the Prefuse project I was able to create a visualization of a table with the x-axis labeled with pit1, pit2 and pit3 and the y-axis with the different values in correct order.

But what I’ve tried for hours is to somehow draw little squares at the corresponding spots (like a red square for car1 where pit1 and value11 “meet”).

How do I do this?

PS: I also would like to know how to improve the y-axis in the following way:

Imagine the lowest value is 2.6 and the highest is 32.0.
Right now the y-axis would start with 2.6 and just label every value (with the same space between the labels regardless of the actual difference) up to 32.0.
What I would prefer is that the labels would start with 0.0 (or 2.0) and then use steps of 5 or so till 35.

Answer 1

You want to visualize your table in a scatter plot with PIT on the x-axis, Value on the y-axis, and Car as the color of the marks (= rectangles).

If I understood you correctly, you see the axis labels but no marks.

You need to add a ColorAction to your action list in order to see the marks. For example:

ColorAction color = new ColorAction("data", VisualItem.STROKECOLOR,
                ColorLib.rgb(100, 100, 255));

Or use a DataColorAction, which allows you to visualize marks in different color depending on Car:

ColorAction color = new DataColorAction("data", "Car", 
                Constants.NOMINAL, VisualItem.FILLCOLOR);

Regarding the y-axis: prefuse draws every label and ignores actual difference, if it cannot read the variable (here: Value) as a double. Please check if it is stored as Double or String.
If you want to start at 0.0 you you can set a range model:

y_axis.setRangeModel(new NumberRangeModel(0, 40, 0, 40)); 

PS: I have written a tutorial for prefuse scatter plots: http://www.ifs.tuwien.ac.at/~rind/w/doku.php/java/prefuse-scatterplot