Vegalite daily line chart wobble - vega

Question

I've run into an issue with vegalite whereby charts exhibit a 'wobbling line' when the line should be straight, and dates are not equally spaced.
Can anyone verify that this is a bug, or am I making a mistake in my spec?
.
I have found that this issue becomes more severe when you increase the number of data points.
To replicate this issue, paste the following spec into the vega lite editor:
{
"description": "",
"data": {
"values": [
{
"date": "2017-01-23",
"value": 100
},
{
"date": "2017-01-24",
"value": 200
},
{
"date": "2017-01-25",
"value": 300
},
{
"date": "2017-01-26",
"value": 400
},
{
"date": "2017-01-27",
"value": 500
},
{
"date": "2017-01-28",
"value": 600
},
{
"date": "2017-01-29",
"value": 700
},
{
"date": "2017-01-30",
"value": 800
},
{
"date": "2017-01-31",
"value": 900
},
{
"date": "2017-02-01",
"value": 1000
},
{
"date": "2017-02-02",
"value": 1100
},
{
"date": "2017-02-03",
"value": 1200
},
{
"date": "2017-02-04",
"value": 1300
},
{
"date": "2017-02-05",
"value": 1400
},
{
"date": "2017-02-06",
"value": 1500
},
{
"date": "2017-02-07",
"value": 1600
}
]
},
"mark": "line",
"encoding": {
"x": {
"field": "date",
"type": "temporal"
},
"y": {
"field": "value"
}
},
"config": [],
"embed": {
"renderer": "canvas",
"actions": {
"export": false,
"source": false,
"editor": false
}
}
}
Edit: Followup - experimenting in Altair, it seems like the date aspect of this is irrelevant. You get the same problem with both of the following code blocks:
import pandas as pd
import numpy as np
from altair import *
s1 = pd.date_range(start="2017-01-23", end="2020-02-07")
s2 = np.arange(1,len(s1)+1)*100
df = pd.DataFrame({"date":s1, "value":s2})
Chart(df).mark_line(
).encode(
x='date',
y='value'
)
and
import pandas as pd
import numpy as np
from altair import *
s1 = np.arange(1,1000,1)
s2 = np.arange(1,len(s1)+1)*100
df = pd.DataFrame({"x":s1, "value":s2})
Chart(df).mark_line(
).encode(
x='x',
y='value'
)
Conversely the following produced a smooth plot (pandas and matplotlib):
%matplotlib inline
df.plot('date', 'value')

Answer 1

The wiggle is caused by an effect of rounding error during the calculation of the pixel coordinates relevant to the data values.
Looking at the vega code produced by vega-lite, one can see the "round": true entries for the defined scales. Changing this to false solves the problem on my screen, and making vega-lite do that, is also possible by adding:
"config": {"scale": {"round" : false}},
instead of the
"config": [],
line in the vega-lite spec.