Customise vega-lite Line Graph Axis Labels - vega-lite

Question

What I want to do is really simple, but I just can't seem to get it right. I have a feeling I'm going to be embarrassed by the answer!
I have a line graph with "attempt" along the x-axis and "grade" along the y-axis, with grade being a number between 0 and 100. I simply want to change the y-axis so that, instead of seeing the raw number, a grade is show, say with 0 - 20 representing "E", 20-40 being "D" etc up to "A" (80-100). How can I do that? I don't want to use discrete values because I want to visually show where within a grade boundary a grade falls. I'm not sure whether I yet want to simply display the grade bands on the line or put them in the middle of their ticks, but just getting somewhere with this would help a lot!
Here is what I've been working with in the vega-lite editor:
{
"$schema": "https://vega.github.io/schema/vega-lite/v3.json",
"data": {
"values": [
{
"attempt": 1,
"score": 30
},
{
"attempt": 2,
"score": 60
},
{
"attempt": 3,
"score": 75
},
{
"attempt": 4,
"score": 58
},
{
"attempt": 5,
"score": 67
}
]
},
"mark": {
"type": "line",
"color": "#22bc9a",
"point": {
"filled": false
}
},
"encoding": {
"x": {
"field": "attempt",
"type": "quantitative",
"axis": {
"grid": false,
"tickCount": 5,
"title": "Attempt"
}
},
"y": {
"field": "score",
"scale": {"domain": [0, 100]},
"type": "quantitative",
"axis": {
"tickCount": 5,
"title": "Grade"
}
},
"opacity": {"value": 0.3}
},
"config": {
"autosize": "fit",
"axis": {
"labelColor": "#bebec8",
"tickColor": "#bebec8",
"titleColor": "black",
"titleFontWeight": "normal",
"titleFontSize": 11,
"labelFont": "Helvetica",
"titleFont": "Helvetica",
"gridOpacity": 0.4,
"gridWidth": 1.5,
"domain": false
},
"view": {
"strokeWidth": 0
}
}
}
Thanks in advance.

Answer 1

What about something like this: I add a dataframe with the grade category, and use this to layer some text. I remove the labels of the axis and so the text acts as if it were the labels of the axis.
The chart looks like this, and here is a link to the editor:
Schema (Note that I did it with Python's Altair, so it may not be canonical Vega-lite, and I did not use your settings either, sorry about that):
{
"$schema": "https://vega.github.io/schema/vega-lite/v2.6.0.json",
"config": {
"view": {
"height": 300,
"width": 400
}
},
"datasets": {
"data-1acee7c5d817865a529b53e022474ce8": [
{
"label": "E",
"x_min": 1,
"y_med": 10
},
{
"label": "D",
"x_min": 1,
"y_med": 30
},
{
"label": "C",
"x_min": 1,
"y_med": 50
},
{
"label": "B",
"x_min": 1,
"y_med": 70
},
{
"label": "A",
"x_min": 1,
"y_med": 90
}
],
"data-8e6359ea3034b8410708361bb10fafd5": [
{
"attempt": 1,
"score": 30
},
{
"attempt": 2,
"score": 60
},
{
"attempt": 3,
"score": 75
},
{
"attempt": 4,
"score": 58
},
{
"attempt": 5,
"score": 67
}
]
},
"layer": [
{
"data": {
"name": "data-1acee7c5d817865a529b53e022474ce8"
},
"encoding": {
"text": {
"field": "label",
"type": "ordinal"
},
"x": {
"field": "x_min",
"scale": {
"zero": false
},
"type": "quantitative"
},
"y": {
"axis": {
"labels": false,
"tickCount": 5,
"ticks": false
},
"field": "y_med",
"type": "quantitative"
}
},
"mark": {
"dx": -15,
"dy": 8,
"size": 15,
"type": "text"
}
},
{
"data": {
"name": "data-8e6359ea3034b8410708361bb10fafd5"
},
"encoding": {
"x": {
"axis": {
"tickCount": 5
},
"field": "attempt",
"title": "Attempt",
"type": "quantitative"
},
"y": {
"field": "score",
"scale": {
"domain": [
0,
100
]
},
"title": "Grade",
"type": "quantitative"
}
},
"mark": {
"point": true,
"type": "line"
}
}
]
}
Using a slighly modified dataframe for the categories (with x_max, y_min and y_max added), you can add another layer with colored rectangles, that can help read the values:
Here is a link to the editor
And here is the schema
{
"$schema": "https://vega.github.io/schema/vega-lite/v2.6.0.json",
"config": {
"view": {
"height": 300,
"width": 400
}
},
"datasets": {
"data-1acee7c5d817865a529b53e022474ce8": [
{
"label": "E",
"x_min": 1,
"y_med": 10
},
{
"label": "D",
"x_min": 1,
"y_med": 30
},
{
"label": "C",
"x_min": 1,
"y_med": 50
},
{
"label": "B",
"x_min": 1,
"y_med": 70
},
{
"label": "A",
"x_min": 1,
"y_med": 90
}
],
"data-39ffbda2b5d5fe96de84d9e308d920ff": [
{
"label": "E",
"x_max": 5,
"x_min": 1,
"y_max": 20,
"y_med": 10,
"y_min": 0
},
{
"label": "D",
"x_max": 5,
"x_min": 1,
"y_max": 40,
"y_med": 30,
"y_min": 20
},
{
"label": "C",
"x_max": 5,
"x_min": 1,
"y_max": 60,
"y_med": 50,
"y_min": 40
},
{
"label": "B",
"x_max": 5,
"x_min": 1,
"y_max": 80,
"y_med": 70,
"y_min": 60
},
{
"label": "A",
"x_max": 5,
"x_min": 1,
"y_max": 100,
"y_med": 90,
"y_min": 80
}
],
"data-8e6359ea3034b8410708361bb10fafd5": [
{
"attempt": 1,
"score": 30
},
{
"attempt": 2,
"score": 60
},
{
"attempt": 3,
"score": 75
},
{
"attempt": 4,
"score": 58
},
{
"attempt": 5,
"score": 67
}
]
},
"layer": [
{
"data": {
"name": "data-39ffbda2b5d5fe96de84d9e308d920ff"
},
"encoding": {
"color": {
"field": "label",
"scale": {
"scheme": "greenblue"
},
"type": "ordinal"
},
"x": {
"field": "x_min",
"scale": {
"zero": false
},
"title": "Attempt",
"type": "quantitative"
},
"x2": {
"field": "x_max",
"type": "quantitative"
},
"y": {
"axis": null,
"field": "y_min",
"type": "quantitative"
},
"y2": {
"field": "y_max",
"type": "quantitative"
}
},
"mark": "rect"
},
{
"data": {
"name": "data-1acee7c5d817865a529b53e022474ce8"
},
"encoding": {
"text": {
"field": "label",
"type": "ordinal"
},
"x": {
"field": "x_min",
"scale": {
"zero": false
},
"type": "quantitative"
},
"y": {
"axis": {
"labels": false,
"tickCount": 5,
"ticks": false
},
"field": "y_med",
"type": "quantitative"
}
},
"mark": {
"dx": -15,
"dy": 8,
"size": 15,
"type": "text"
}
},
{
"data": {
"name": "data-8e6359ea3034b8410708361bb10fafd5"
},
"encoding": {
"x": {
"axis": {
"tickCount": 5
},
"field": "attempt",
"title": "Attempt",
"type": "quantitative"
},
"y": {
"field": "score",
"scale": {
"domain": [
0,
100
]
},
"title": "Grade",
"type": "quantitative"
}
},
"mark": {
"point": true,
"type": "line"
}
}
]
}

Answer 2

To get it working, I first had to change the encoding of the x and y axis to be ordinal. Then I mapped your input data values to letter grades before creating the schema:
//replace every score value with correct letter grade
values.forEach(datapoint => {
if(datapoint.score > 90) {
datapoint.score = "A";
} else if(datapoint.score > 80) {
datapoint.score = "B";
} else if (datapoint.score > 70) {
//so on...
}
});
Here is a working example in the vega-lite editor:
Line Chart with Ordinal Values
Here is the schema:
{
"$schema": "https://vega.github.io/schema/vega-lite/v3.json",
"data": {
"values": [
{
"attempt": 1,
"score": "F"
},
{
"attempt": 2,
"score": "D"
},
{
"attempt": 3,
"score": "C"
},
{
"attempt": 4,
"score": "F"
},
{
"attempt": 5,
"score": "D"
}
]
},
"mark": {
"type": "line",
"color": "#22bc9a",
"point": {
"filled": false
}
},
"encoding": {
"x": {
"field": "attempt",
"type": "ordinal",
"axis": {
"title": "Attempt"
}
},
"y": {
"field": "score",
"type": "ordinal",
"axis": {
"title": "Grade"
}
},
"opacity": {"value": 0.3}
},
"config": {
"autosize": "fit",
"axis": {
"labelColor": "#bebec8",
"tickColor": "#bebec8",
"titleColor": "black",
"titleFontWeight": "normal",
"titleFontSize": 11,
"labelFont": "Helvetica",
"titleFont": "Helvetica",
"gridOpacity": 0.4,
"gridWidth": 1.5
},
"view": {
"strokeWidth": 0
}
}
}