ChartJS – why is there a visual difference when displaying the same dataset alone or with more datasets?

I’ll recommend you to set a minimum and maximum display value to the Y Axis like:

 yAxes: [{
  gridLines: {
    display: true,
    drawTicks: false,
  },
  ticks: {
    min:0,
    max: 25
  }
}]

If its not done, it auto-scales to the minimum and maximum value of the datasets, and it can be different in each display mode.