Get In between points of an Ordinal Scale Axis in d3 - d3.js

Question

I am having a hard time in finding the points in between the given X scale domain.
For eg. I have a bar chart having its X axis values as
[Bob,Robin,Anne,Mark,Joe,Eve,Karen,Kirsty,Chris,Lisa,Tom,Stacy,Charles,Mary]
The above values are used to plot the x-axis.I have used scaleBand method since the values are discrete.
var x = d3.scaleBand()
.range([0, width])
.padding(0.1);
x.domain(data.map(function(d) { return d.salesperson; }));
My Aim is to find points in between x scale from the given points,i.e
Is there any way that we can locate a point between Bob,Robinor Robin,Anne, So that it would be possible for me to plot a point provided a Y axis value is given.
I have added a plunker which illustrates bar chart having the mentioned x-axis.I need to plot a point on the chart having X value point between Bob,Robin
and Y value as 30(any value on Y axis)

Answer 1

Provided that Robin is the discrete value just after Bob, you can use a combination of bandwidth() and step():
x("Bob") + x.bandwidth() + (x.step()-x.bandwidth()) / 2
Ugly, but it works. Talking about ugliness, if you want an uglier math, you can also use a combination of step() and paddingInner():
x("Bob") + x.step() * (1 - x.paddingInner() / 2))
Here is a demo, I put the point at y(10) so you can better see that it will be right between the "Bob" and "Robin" bars:
var margin = {top: 20, right: 20, bottom: 30, left: 40},
width = 960 - margin.left - margin.right,
height = 500 - margin.top - margin.bottom;
// set the ranges
var x = d3.scaleBand()
.range([0, width])
.padding(0.1);
var y = d3.scaleLinear()
.range([height, 0]);
// append the svg object to the body of the page
// append a 'group' element to 'svg'
// moves the 'group' element to the top left margin
var svg = d3.select("body").append("svg")
.attr("width", width + margin.left + margin.right)
.attr("height", height + margin.top + margin.bottom)
.append("g")
.attr("transform",
"translate(" + margin.left + "," + margin.top + ")");
var data = [
{
"salesperson": "Bob",
"sales": 33
},
{
"salesperson": "Robin",
"sales": 12
},
{
"salesperson": "Anne",
"sales": 41
},
{
"salesperson": "Mark",
"sales": 16
},
{
"salesperson": "Joe",
"sales": 59
},
{
"salesperson": "Eve",
"sales": 38
},
{
"salesperson": "Karen",
"sales": 21
},
{
"salesperson": "Kirsty",
"sales": 25
},
{
"salesperson": "Chris",
"sales": 30
},
{
"salesperson": "Lisa",
"sales": 47
},
{
"salesperson": "Tom",
"sales": 5
},
{
"salesperson": "Stacy",
"sales": 20
},
{
"salesperson": "Charles",
"sales": 13
},
{
"salesperson": "Mary",
"sales": 29
}
];
// format the data
data.forEach(function(d) {
d.sales = +d.sales;
});
// Scale the range of the data in the domains
x.domain(data.map(function(d) { return d.salesperson; }));
y.domain([0, d3.max(data, function(d) { return d.sales; })]);
// append the rectangles for the bar chart
svg.selectAll(".bar")
.data(data)
.enter().append("rect")
.attr("class", "bar")
.attr("x", function(d) { return x(d.salesperson); })
.attr("width", x.bandwidth())
.attr("y", function(d) { return y(d.sales); })
.attr("height", function(d) { return height - y(d.sales); });
// add the x Axis
svg.append("g")
.attr("transform", "translate(0," + height + ")")
.call(d3.axisBottom(x));
// add the y Axis
svg.append("g")
.call(d3.axisLeft(y));
var pointBetween = svg.append("circle")
.attr("r", 2)
.attr("cy", y(10))
.attr("cx", x("Bob") + x.bandwidth() + (x.step()-x.bandwidth())/2)
.bar { fill: steelblue; }
<script src="//d3js.org/d3.v4.min.js"></script>