[Solved]-Highcharts: make y-axis extremes the same for two data series with different units-Highcharts

there is an option to achieve that. you can link one axis to another via linkedto property.

from api:

linkedto : number

index of another axis that this axis is linked to. when an axis is linked to a master axis, it will take the same extremes as the master, but as assigned by min or max or by setextremes. it can be used to show additional info, or to ease reading the chart by duplicating the scales.

http://api.highcharts.com/highcharts/yaxis.linkedto

example: http://jsfiddle.net/n8tokqru/

    yaxis: [{

    }, {
        linkedto: 0
    }],