score:15

Accepted answer

You'll have to specify a tooltip formatter yourself (see documentation):

tooltip: {
  formatter: function() {
    return 'The value for <b>'+ this.x +
           '</b> is <b>'+ this.y +'</b>';
  }
},

there you can just show the x-values if you like or your custom text.
Hope that helps.

score:0

if you want to format the second line of the tool tip, but leave the x-axis label name alone you can use point format instead of formatter.

tooltip: {
     pointFormat: 'The value is point.y'
},

score:2

I have modified the DEMO and created a new DEMO here

To customize the text displayed in the tooltip, you should use it like:

Highcharts.chart('container', {

    title: {
        text: 'Solar Employment Growth by Sector, 2010-2016'
    },

    subtitle: {
        text: 'Source: thesolarfoundation.com'
    },
........
.....
....

    tooltip: {
      formatter: function() {
        return '<strong>X value: </strong>'+ this.x;
      }
    },
}); 

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