The problem here is that Time values include a date as well, in other words, there is way to pass "1:00 pm" to the chart you need to pass a date too so that 1:00 pm of today is different than 1:00 pm of tomorrow. This is regardless of the labels format. Even thoug you are displaying only the time it doesn't mean that there is not date associated with it.
So the datatype in the chart is DateTime regardless o your label format which is used just for displaying. As such, there is no concept of a negative datetime since there is no zero (there is but it doesn't mean anything).
It looks that what you are looking for is a TimeSpan datatype. We do not support TimeSpans in this version althought this is something we will have in the next version. What you can do now is "trick" the chart. Here is how you can do it:
1) You will convert your timespan values to integers. You can use the TotalMilliseconds for this. These will end up being possitive and negative numbers,
2) You will leave the X-Axis as numeric.
3) Capture the GetAxisLabel event and convert the label about to be displayed (numeric value. eg. 1000,-1500,etc.) back to a TimeSpan and set the label to be this string.