I'm trying to make an interface that lets a user upload CSV files and plots these using plotly, only using javascript and obviously the plotly library. I'm close, but my suspicion is that there's an issue with the asynchronous reading of the csv files. As you can probably see, I'm relatively new to javascript, so any feedback is welcome. I cannot however use any other libraries or packages plotly.

The problem is that the resulting figure only shows the initialized values (1).

EDIT: The heatmap function works on test data, or if I modify specific elements of the data_y object, just not when I update the information from the file.

1. There's a button that allows uploading of the csv files. On event this code triggers:

<script>
let picker  = document.getElementById('picker');

picker.addEventListener('change', event => {0
    file_list  = event.target.files;
    var fig_y  = [];

    for (let i = 0 ; i< file_list.length ; i++){
    if(file_list[i].name == (".DS_Store")){continue}
    else {
    var ready = read_data(file_list[i]); 
    fig_y.push(ready);
    }
    }   
    console.log(fig_y);
    plot_heatmap(fig_y);
}
);
</script>

2. The data is read using this code.

<script>
function read_data(input){
    var xs = 1212;               // length of the data
    file_contents = [];
    var data_y    = Array(xs).fill(1);
    let file      = input;
    
    let reader    = new FileReader();
    reader.readAsText(file);
    reader.onload   = function(){
    file_contents = reader.result.split('\n');

// open the data file. First two lines contain a description of the data.
    for (let j  = 2 ; j<file_contents.length-1 ; j++) {
// the relevant data is the third number in the column
    var nr = file_contents[j].split(",").map(Number)[2];  
    data_y[j-2] =   nr;
    }
    }
    return data_y;
}
</script>

3. the code that makes the plotly heatmap.

<script>
function plot_heatmap(data_z){

    var data = [
      {
           z: data_z,
        type: 'heatmap'
      }
];
Plotly.newPlot('raw_data', data);
    };
</script>