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Line Transition
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This example is the second of three in the Path Transitions tutorial; see the previous example for context.

The desired pairing of numbers for path interpolation is like this:

``````M x0, y0 L x1, y1 L x2, y2 L x3, y3 L xR, y4
↓   ↓    ↓   ↓    ↓   ↓    ↓   ↓
M xl, y0 L x0, y1 L x1, y2 L x2, y3 L x3, y4
``````

Where `xl` is some negative value off the left side, and `xr` is some positive value off the right side. This way, the first point ⟨x0,y0⟩ is interpolated to ⟨xl,y0⟩; meaning, the x-coordinate is interpolated rather than the y-coordinate, and so the path appears to slide off to the left. Likewise, the incoming point ⟨xr,y4⟩ is interpolated to ⟨x3,y4⟩.

While you could write a custom interpolator and use transition.attrTween to achieve this, a much simpler solution is to interpolate the transform attribute rather than the path. This way, the shape of the path remains static while the it translates left during the transition.

Immediately prior to the transition, the path is redrawn as follows:

``````M x0, y0 L x1, y1 L x2, y2 L x3, y3 L xr, y4
``````

Then, a transform transition is applied:

``````translate(0,0)
↓
translate(xl,0)
``````

This causes the path to slide left. A clip path is used so the path is not visible outside of the chart body.

Note that for charts with spline interpolation, you’ll need to crop the visible part of the line by an extra point, so that the change in tangent is not visible; see the next example.

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### pepijnverburg commented Apr 4, 2017

Hi,

Quick addition for people who might also have problems with the translate reset not being executed properly, because the translate transition is still in progress (might happen when you are dealing with varying frame rates or data intervals). Took me a fair while and significant debugging to figure it out.

Make sure you reset the transition on the selection when resetting the transform:

``````d3.select(this)
.attr("d", line)
.transition().duration(0)
.attr("transform", null);
``````

How to modify this example to get multi-series chart, for example by adding a 2nd data array?
Now it takes let's say `[11,15,30,1,10,20]` and draws a line with it and how to add an example array of `[33,35,21,40,22,2]` that would draw another line?

### sampaioletti commented Jun 23, 2017

I forked the code pen from @Atticweb and updated to v4. Everything seems to be working properly, but I'm a little unhappy with the transition of the axis text, it doesn't respect the easing (jumpy), but the ticks transition smoothly. Any ideas?

https://codepen.io/sampaioletti/pen/eREyQZ

I'm pretty new to d3, but wanted to use this same idea in a similar project that requires v4.

(sorry if you get this twice I should have posted here in the first place probably not on bjorngi's gist)

### alexsmartens commented Mar 24, 2018

Has anyone implemented something similar in react-native? If so, would you mind sharing your experience?

### Yaweii commented May 30, 2018

@alexsmartens I am trying to build something similar in react-native as well. Did you get any chance to figure this out yet?

### NimChimpsky commented Jan 14, 2021

How can people use this knowing that the tick function is called by an exponentially number of growing of callees ? It will just grind user's cpu/browser into the ground.

### NimChimpsky commented Jan 14, 2021

``````.transition()
.duration(500)
.ease("linear")
.attr("transform", "translate(" + x(-1) + ",0)")
.each("end", tick);
``````

.each(,) runs a new function "tick" for each point after each iteration, so it'll exponentially grow per cycle. I left this open on a second tab and it ran my CPU into the ground.

which therefore makes this example completely invalid without a significant refactoring, which everyone else seems to be happily ignoring ... I don't understand.

### nateblain commented Sep 1, 2021

@NimChimpsky I agree.