Avoid re-initialize values in javascript
You could remove the need for your xAxis variable through using the index when mapping:

vm.pnLreportingPlotData.push(plotModel.map(function (o, i) {
  return { x: i + 1, y: o.PnLreporting }
}

vm.var95Plotdata.push(plotModel.map(function (o, i) {
  return { x: i + 1, y: o.VaR95Total }
}

vm.var99PlotData.push(plotModel.map(function (o, i) {
  return { x: i + 1, y: o.VaR99Total }
}


Array.prototype.map provides three arguments, currentValue (o), index, and array (which isn't needed in our example)

N.B. you did not need to keep declaring xAxis after you had done so the first time (no need for the multiple var declarations, they could simply become xAxis = 1)

While the above answers your question of how to avoid redeclaring xAxis, I've added a few additional comments below:

Naming

You could possibly benefit from using more descriptive variable names. It is not immediately obvious what variables such as o represent. Likewise, i could be changed to index if you found it helped readability.

Consistency:


var95Plotdata uses lowercase 'd' for 'data', however var99PlotData capitializes it (is this a typo?)
VaR95Total capitalizes both 'V' and 'R' - can this be changed to be consistent with your view model or is this coming from an external api?


Extracting logic

Since the logic is almost identical besides property names, you could extract the logic if you wanted:

function getPlotData(property) {
  // Ideally renaming `o` to whatever it represents
  return plotModel.map(function (o, index) {
    return { x: index + 1, y: o[property] }
  }))
}

vm.pnLreportingPlotData.push(getPlotData('PnLreporting'))
vm.var95Plotdata.push(getPlotData('vaR95Total'))
vm.var99PlotData.push(getPlotData('VaR99Total'))