Hello Peter.

Like you mentioned, that is the default behavior of the chart engine, but you should be able to add the labels using our wpDataCharts callbacks. 

• Every chart exposes several options that customize its look and feel. Charts usually support custom options appropriate to that visualization. wpDataChart callbacks allow adding options that are available in Google Charts API, Highcharts API and Chart.js API. 

• All necessary resources are available in charts engines API (depends on which one you use). Every engine has a different approach to chart settings. In wpDataChart callbacks you have to adopt those settings to the wpDataChart object (you can take a look at examples for each engine in our documentation, and also in Support help center). Huge number of examples for any area of programming can be found on stackoverflow.com (typing your problem in google and at the end add "site: stackoverflow.com" and google will search only that website). Also, a lot of examples of charts, chart settings and customization can be found on jsffidle.net (typing in google for example "line chart highcharts jsffidle")
• Please note that using hooks or wpdatatable and wpDataChart callbacks requires certain level of programming skills and included support refers only to advice.

By looking at their APIs a bit, I can point you to stick with pointLabels. This doesn't work, but I hope it will point you in the right direction:

<script type="text/javascript">
jQuery(window).load(function(){
    if( typeof wpDataChartsCallbacks == 'undefined' ){ wpDataChartsCallbacks = {}; }
    wpDataChartsCallbacks[2] = function(obj){
        obj.options.options.scales.yAxes[0].pointLabels = {
            show: true,
            fontSize: 20
        }
    }
});
</script>

I believe you're looking for something like this, so please check Stack Overflow, and other resources.

Best regards.