Calculations in vega-lite - vega

Question

is there a way to dynamically calculate growth rates in Vega-Lite.
For example:
[
{"date": "1/1/2020", "b": 27},
{"date": "1/2/2020", "b": 30},
{"date": "1/3/2020", "b": 33}
]
How could I create data (and a chart) that shows the daily +3 (or the ~+10%)?
Edit: Thanks for the answer, #jakevdp.
Should have outlined the added complexity earlier; apologies: I need to aggregate prior to calculating changes. See below for the data and my attempt (dates seem offset and last date's drop doesn't make sense.
[Vega Editor][1]
{
"data": {
"values": [
{"date": "2020-01-01", "country": "CHN", "count": 0},
{"date": "2020-01-02", "country": "CHN", "count": 2},
{"date": "2020-01-03", "country": "CHN", "count": 4},
{"date": "2020-01-01", "country": "GER", "count": 0},
{"date": "2020-01-02", "country": "GER", "count": 2},
{"date": "2020-01-03", "country": "GER", "count": 4},
{"date": "2020-01-04", "country": "GER", "count": 6}
]
},
"transform": [
{
"aggregate": [{"op": "sum", "field":"count", "as":"daily_count"}],
"groupby": ["date"]
},
{
"window": [
{"op": "lead", "field": "daily_count", "as": "daily_count_tomorrow"}
]
},
{"filter": "isValid(datum.daily_count_tomorrow)"},
{"calculate": "datum.daily_count_tomorrow - datum.daily_count", "as": "change"}
],
"mark": "bar",
"encoding": {
"x": {"type": "ordinal", "field": "date", "timeUnit": "yearmonthdate"},
"y": {"type": "quantitative", "field": "change"}
}
}
[1]: https://vega.github.io/editor/#/url/vega-lite/N4KABGBEAmCGAutIC4yghSA3WAbArgKYDOKYA2uBhMDAoWZAEwAMrAtCwIydeQA0UAMYB7fADt4AJwCejAMIAJAHIDhYyWRYBfflWq048BqmZsWvTkzWRRE6XNNLVg2xvhkmu-RkP1GrBzcnADMNnaSsgoq4e5kACzePjR0xgHmltyx9lGmAOIAogBK2ZqoOnrUKUYmUIEWwWylDoyFJa4RHqhelVV+aab1mWEd7rlQbc0J3lVoqbVmQTws8c3jkJOj9mQAbNo+ALpU3pDSsOLEAGYiUgC2ZJQGyVCwAOavUoSv-qjktCIAB0YxHw91clwAloRcNAUG5tq5YKRkHQIbgZAB9TqQbQHXrUSAfMQAgBGjgo80gR2oM18z0gAHcIeJoCIGQ9nilAYxcIRYLDwVCYYw4GjMdjEcioKL0Vj3Bj4CJbjcpGycc9qRhaSlIbhjFJGBDiAA1PAQ6AACiMoIAdDLxfLFcqpKqGQBKHH4uZCPBCfC4H7ShC2+1y+wKpUqtlgdhga23O2wMVhzSSxhCAAW51eDH2EDxICokFusCkAGtGCTSwIi4RxKJoMzXmR0BhIAAPFunGQAhY3RviPA2SHQ2GmGo2eAQ26EACq4ghXSgMj5dxEkgzE+1y678B7CwAjvhzlPEFOsAxBaP01nxDn1RB9togA

Answer 1

Yes, you can do this using the window transform with the lead or lag operation. For example (vega editor):
{
"data": {
"values": [
{"date": "2020-01-01", "b": 29},
{"date": "2020-01-02", "b": 30},
{"date": "2020-01-03", "b": 32},
{"date": "2020-01-04", "b": 31},
{"date": "2020-01-05", "b": 34}
]
},
"transform": [
{"window": [{"op": "lead", "field": "b", "as": "b1"}]},
{"filter": "isValid(datum.b1)"},
{"calculate": "datum.b1 - datum.b", "as": "change"}
],
"mark": "bar",
"encoding": {
"x": {"type": "ordinal", "field": "date", "timeUnit": "yearmonthdate"},
"y": {"type": "quantitative", "field": "change"},
"color": {
"condition": {"test": "datum.change > 0", "value": "green"},
"value": "red"
}
}
}