How to calculate y value for given x using d3.js

Question

I have defined a line generator with d3.js as follows:

var line = d3.svg.line()
    .interpolate("monotone")
    .x(function(d) {return x(d.date); })
    .y(function(d) {return y0(d.visits); });

The data is read from a csv with the following format:

date,visits
12/08/12,1
13/08/12,0
14/08/12,0
15/08/12,33
16/08/12,28

The csv file is loaded into data , and parsed as:

data.forEach(function(d) {
    d.date = d3.time.format("%d/%m/%y").parse(d.date);
    d.visits = +d.visits;
});

and added to the document with:

svg.append("path")
    .datum(data)
    .attr("class", "line")
    .attr("d", line)

Elsewhere in my script, I need to work out what the y value is at a particular date. So for example, I might want to get ay value for the line where the date is 15/08/12 (equivalent to y0(33) ). How can I do this?

Answer 1

You can use bisect to find the date in your array and then call the y0 function. The code would look something like this (taken pretty much straight from the documentation):

var bisect = d3.bisector(function(d) { return d.date; }).right;
...   
var item = data[bisect(data, new Date(2012, 8, 15))];
y0(item.visits);

Note that this approach requires your dates to be sorted, which they are in your example data.

Answer 2

Edit

To piggy back off of Lars' example, with a minor tweak to avoid -1 hacks:

var bisect = d3.bisector(function(d) { return d.date; }).left;
...   
var item = data[bisect(data, new Date(2012, 8, 15))];
y0(item.visits);

Original Post

rudivonstaden, you can use the .left function attached to the bisector instead of the .right in Lars' example -- that way you do not have to do the -1 hack that you did.