Raspberry Pi Pico Tips and Tricks

Tuesday 8 November 2016

Change a line chart into a scatter plot in d3.js v4

The following post is a section of the book 'D3 Tips and Tricks v4.x'.  The entire book can be downloaded in pdf format for free from Leanpub or you can read it online here.
Since this post is a snapshot in time. I recommend that you download a copy of the book which is updated frequently to improve and expand the content.

Change a line chart into a scatter plot

Confession time.
I didn’t actually intend to add in a section with a scatter plot in it because I thought it would be;
  1. tricky
  2. not useful
  3. all of the above
I was wrong on all counts.
I did want to have a scatter plot, because I wanted to display tool tips, but this is too neat to ignore. It was literally a 5 minute job, 3 minutes of which was taken up by going to the d3 gallery on the wiki and ogling at the cool stuff there before snapping out of it and going to the scatter plot example.

All you need to do is take the simple graph example file. The full code for this can be found on github or in the code samples bundled with this book (simple-graph.html and data.csv). A live example can be found on bl.ocks.org.
We then slot the following block in between the ‘Add the valueline path’ and the ‘add the x axis’ blocks.

  // Add the scatterplot
      .attr("r", 5)
      .attr("cx", function(d) { return x(d.date); })
      .attr("cy", function(d) { return y(d.close); });

And you will get…
A scatter plot! (with a line)

The full code for this graph can also be found on github or in the code samples bundled with this book (scatterplot.html and data.csv). A live example can be found on bl.ocks.org.
I deliberately put the dots after the line in the drawing section, because I thought they would look better, but you could put the block of code before the line drawing block to get the following effect;
A scatter plot with the line in front of the dots

(just trying to reinforce the concept that ‘order’ matters when drawing objects :-)).
You could of course just remove the line block all together…
A scatter plot without the line this time

But in my humble opinion it loses something.
So what do the individual lines in the scatter plot block of JavaScript do?
The first line (svg.selectAll("dot")) essentially provides a suitable grouping label for the svg circle elements that will be added. The next line associates the range of data that we have to the group of elements we are about to add in.
Then we add a circle for each data point (.enter().append("circle")) with a radius of 5 pixels (.attr("r", 5)) and appropriate x (.attr("cx", function(d) { return x(d.date); })) and y (.attr("cy", function(d) { return y(d.close); });) coordinates.

There is lots more that we could be doing with this piece of code (check out the scatter plot example) including varying the colour or size or opacity of the circles depending on the data and all sorts of really neat things, but for the mean time, there we go. Scatter plot!

The post above (and heaps of other stuff) is in the book 'D3 Tips and Tricks v4.x' that can be downloaded for free (or donate to encourage further development :-)).

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