DateAxis and XYBarChart update

Just the other day Gerrit Grunwald (@hansolo_) tweeted he made a DateAxis which supports JSR310 (date/time) api, and which is based on my work and also on Christian Schudt and Diego Cirujano-Cuesta work (they created a DateAxis which accepts DateTime). He later added that he would eventually create a blog post about it if he had time (you should stay tuned his posts are usually high quality). This reminded me that a blog post about my DateAxis and XYBarChart is long overdue since I’ve made a lot of changes since my last posts on the subject. Besides the changes I also wanted to explain my implementation of the DateAxis and why I’m sticking with the design (code design not artistic design) choices I’ve made.

Design decisions while creating DateAxis

While creating DateAxis I had two choices, either extend from Axis or from ValueAxis. Extending from Axis would allow me to accept DateTime or the new date formats from JSR310, while extending from ValueAxis would guarantee the presence of methods like setLowerBound and setUpperBound which would allow a user to filter or zoom in on specific dates, a fairly common use case, but would mean programmers would first need to convert the date to a number before adding it to the chart. Of course you can add filtering methods yourself while extending from Axis but a component that does zooming on charts and is expecting a ValueAxis (which guarantees setUpperBound and setLowerBound) wouldn’t work with that implementation.

Even though at first I was thinking that extending from Axis would be better, I’m now more convinced that the current solution – extending from ValueAxis, is a better one, for the reasons expressed earlier, and the draw back of having to convert the date to a number isn’t such a big thing.

I think it would be best if the JavaFX charts API would allow one to separate the position of a value in an axis from the value itself I think it would be best for setLowerBound and setUpperBound to be mandatory methods on Axis, but has things are right now you have to take in consideration the two drawbacks mentioned earlier before starting to implement a new axis.

Changes made to DateAxis and XYBarChart

I’m afraid I’ve probably forgotten about most of the changes I’ve made, anyway here’s what I recall:

DateAxis

  1. After changing range (by zooming in for instance) the tick marks interval stayed the same. Now they are recalculated.
  2. Added a bigger set of tick ranges to accommodate a higher range of dates.

XYBarChart (this previous post explains what this is)

  1. On some occasions depending on the data you add to the chart, it may not render any bars even though there is data to be displayed. This is a bug that exists in javafx BarChart and was passed on to this chart since there was some copying of code.

Today, I also added one more enhancement to the DateAxis which was on the pipeline of changes I wanted to make. Basically the format of the dates will change depending on the range of values that is being shown: for instance, if each tick mark represents more than a month than only the month and year of that tick mark is shown otherwise the day, month and year is displayed. A similar thing happens if the range of tick marks is bigger than one year, in that case only the corresponding year is shown. The following pictures show the before and after this change.

beforeChange8

XYBarChart with a DateAxis – before change

afterChange8

XYBarChart with a DateAxis – after change

This is just a small detail that makes for an easier scanning of the chart by the user.

If you want to join me, in adding more features to the charts API, you are welcome. My eventual objective would be for this to be integrated in the JavaFX API (at least the DateAxis) although I don’t know if that would be possible.

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XYBarChart for javafx

One new addition to the FXCharts repo that I’ve created on github. This time it’s a XYBarChart, that is a bar chart that doesn’t have the limitation of the SDK BarChart which only allows you to have a CategoryAxis/ValueAxis pair of axes.So in other words, for example,  you can’t set a pair of axes if their of type DateAxis/ValueAxis. And as another consequence of these you can’t easily add zoom capabilities to a BarChart.

These type of chart wasn’t so easy to create was one might initially expect. Copy pasting the code from BarChart and altering the significant bits is not enough since BarChart relies on several package private methods from XYChart.

You can grab it from: https://github.com/dukke/FXCharts. If you have patches, features, fixes for this library please participate. Let’s create a better charts API.

 

DateAxis for javafx

A somewhat common request for people working with the charts API in JavaFX is a DateAxis. Essentially that would be an Axis that would be able to show dates.

I’ve hacked up a quick implementation of one, you can get it from github: https://github.com/dukke/FXCharts

It takes the time value of dates and displays them using the format given defined by “TICK_UNIT_FORMATTER_DEFAULTS”.

It’s not really a very elegant solution but it works. I’ll try to give a hand to Diego Cirujano-Cuesta and Christian Schudt who are working in a more evolved DateAxis that takes up Dates instead of their time value.

This implementation was done under Java8. It might work with JavaFX 2.2, I don’t know.