If you want to give some data to initialize the chart, you can assign it:

def mount(_params, _session, socket) do
  {
    :ok,
    assign(socket,
      temp_chart_data: %{
        labels: ["foo", "bar"],
        values: [21, 12]
      }
    )
  }
end

Then pass it to your chart as data-whatever-you-like in the render/html.leex

<canvas id="temp-chart-canvas"
        phx-hook="TempChart"
        data-chart-data="<%= Jason.encode!(@temp_chart_data) %>">

and then you can use dataset.whateverYouLike in your hook to initialize the chart:

Hooks.TempChart = {
  mounted() {
    const { labels, values } = JSON.parse(this.el.dataset.chartData)
    this.chart = new TempChart(this.el, labels, values)
    ...
    handleEvent...

The issue is that you have a race condition. handle_params is run before the javascript on the page has fully rendered. Instead of pushing it in handle_params, instead have the hook JS, on mount, push a “get me the data” event to the live view, which you do in a handle_event clause. Then in that, you send the data.