Dashboard UI Chart Legend Font SIze

Hi!

Sorry for digging out this old thread, but I'm interested in customising my chart. I followed the suggestions here but I'm stuck. Maybe an image first:
image
Depending on the multistate switch I display the data. But I want to adjust the x-axis labels depending on the time interval. For the case in the image I'd like to have tick labels every 10 minutes. I tried:

{
    "options": {
        "scales": {
            "xAxes": [
                {
                    "type": "time",
                    "time": {
                        "displayFormats": {
                            "millisecond": "HH:mm",
                            "second": "HH:mm",
                            "minute": "HH:mm",
                            "hour": "HH",
                            "day": "MMM D"
                        },
                        "unit": "minute",
                        "stepSize": 10
                    },
                    "ticks": {
                        "display": true,
                        "fontSize": 11,
                        "maxRotation": 50
                    },
                    "gridLines": {
                        "display": false,
                        "drawBorder": false,
                        "drawOnChartArea": true,
                        "drawTicks": true,
                        "tickMarkLength": 5
                    }
                }
            ]
        },
        "legend": {
            "display": true
        }
    }
}

How do I tell the chart to show me tick labels every 10 minutes? I thought that

"stepSize": 10

was the option I needed, but it doesn't have an effect. Even if I use 1 or 1000. There must be something I didn't quite understand in the docs, can somebody give me a hint please? Thanks in advance!