The current value formatting logic is a bit tricky and can be described as a set of fallback options with the following orders and priorities:

Formatter for X axis labels (used for non-multilayer axis):

1. current axis axisSpec.labelFormat
2. current axis axisSpec.tickFormat
3. toString

Formatter for the Y axis labels:

1. current axis axisSpec.labelFormat
2. current axis axisSpec.tickFormat
3. the first seriesSpec.tickFormat for the relative groupId
4. toString

Formatters for the tooltip header:

1. Settings.tooltip.headerFormatter
2. the last X axisSpec axisSpec.tickFormat <--- to fix
3. toString

Formatters for the tooltip value:

1. current series seriesSpec.tickFormat
2. the last Y axisSpec axisSpec.tickFormat <--- to fix
3. toString

Formatters for the legend value:

1. current seriesseriesSpec.tickFormat
2. the last Y axisSpec axisSpec.tickFormat <--- to fix
3. toString

Formatters for the bar data values:

1. the last Y axisSpec axisSpec.tickFormat <--- to fix
2. the first seriesSpec seriesSpec.tickFormat <--- to fix
3. toString

This brings in a lot of instability and lack of clarity when defining formatters for the various label in the chart.
I'd like to propose the following:

1. better naming for the various formatters: something along the following lines labelTickFormatter to format axis tick labels, barValueLabelFormatter for bar data values, legendExtraValueLabelFormatters, tooltipValueLabelFormatter and tooltipHeaderValueLabelFormatter
2. create a clear and logical fallback when not specified.