You can set the minimum and maximum values for your line chart Y-Axis as part of the widget configuration page, rather than as part of your dataset's schema.
Example custom Y-Axis
Toggle Chart Options to bring up the Y-Axis options for your line chart.
It's not possible to manually specify the interval used between these values on the Y-Axis. This is set automatically based on the datapoints sent to the chart and the size of the widget used.