Wrong display with a custom scale when all values are positive

[ronenl]

I configured the options like this:
$scope.chartOptions = {
scaleOverride: true,
scaleStartValue: -100,
scaleSteps: 10,
scaleStepWidth: 20
};

When the chart has mixed positive and negative values the chart looks OK,
but when all the values are positive the chart looks like this:

the values are:
[
{
"StartDate": "/Date(1433106000000)/",
"EndDate": "/Date(1433192400000)/",
"StartDateFormatted": "1",
"EndDateFormatted": "2",
"Performance": 3.37662337662
},
{
"StartDate": "/Date(1433192400000)/",
"EndDate": "/Date(1433278800000)/",
"StartDateFormatted": "2",
"EndDateFormatted": "3",
"Performance": 6.70125378297
},
{
"StartDate": "/Date(1433278800000)/",
"EndDate": "/Date(1433365200000)/",
"StartDateFormatted": "3",
"EndDateFormatted": "4",
"Performance": 1.0989010989
},
{
"StartDate": "/Date(1433365200000)/",
"EndDate": "/Date(1433451600000)/",
"StartDateFormatted": "4",
"EndDateFormatted": "5",
"Performance": 5.05737356566
},
{
"StartDate": "/Date(1433451600000)/",
"EndDate": "/Date(1433538000000)/",
"StartDateFormatted": "5",
"EndDateFormatted": "6",
"Performance": 0.76335877863
},
{
"StartDate": "/Date(1433538000000)/",
"EndDate": "/Date(1433624400000)/",
"StartDateFormatted": "6",
"EndDateFormatted": "7",
"Performance": 4.50077599586
},
{
"StartDate": "/Date(1433624400000)/",
"EndDate": "/Date(1433710800000)/",
"StartDateFormatted": "7",
"EndDateFormatted": "8",
"Performance": 5.26315789474
},
{
"StartDate": "/Date(1433710800000)/",
"EndDate": "/Date(1433797200000)/",
"StartDateFormatted": "8",
"EndDateFormatted": "9",
"Performance": 0.25839793282
}
]

The values are starting from the bottom of the chart instead of starting from the zero line.