Help Vega-Lite line graph

I’m trying to create a line chart representing a cash flow with Vega Lite.
In my main table I have record that are cash transactions with a date and an amount (either positive or negative).
I want to display my bank balance for each month: in February I want to have January + February and not only February.
Does anyone has an idea on how to do it ?

Here is my vega code:

  "$schema": "",
  "title": "Solde mois par mois",
  "width": "container",
  "height": "container",
  "mark": {
      "type": "line",
      "point": true,
      "tooltip": true
  "encoding": {
    "x": {
      "field": "TECH Cash flow date",
      "title": "Mois",
      "timeUnit": "month",
      "type": "nominal"
  "y": {
      "field": "Cash Flow (pondéré)",
      "aggregate": "sum",
      "title": "Solde bancaire (€)",
      "type": "quantitative"

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