The chart shows the mid-price of the spread. When looking at the graph always remember it is indicative, the price + spread can always go beyond what the graph displays – graphs always state indicative pricing – outside hours the spread widens and obviously this doesn’t show up on graph. Sounds like the spread got you…. Always set your stop to your level from a graph plus spread and a bit, and be prepared to be unlucky. And do remember to check the price on the ticket…
Having said that I suspect that one reason that spread betting providers love volatility is that trading positions get taken out on spikes in prices (which only last a minute or so, and display as long tails on candlestick charts) which trigger stops en masse. Although stop loss order can protect your from big losses, ‘stop hunting’ can get you to accumulate lots of small losses over time, whilst taking you out of potential winning trades early at a loss.
My suggestion is to widen stops when using spreadbets. If you see it happening often switch providers, or widen stops and reduce bet size, and do familiarise yourself with the volatility of the market you are trading. I review my stops every night, and sometimes during the day, too. Part and parcel of using spread betting to trade. Quite often I loosen stops in the evening and then tighten them again the following morning especially if things are choppy. I have learnt the hard way that this avoids getting stopped out on a silly spike or wide spread. Generally speaking I keep stops loose for shares I am holding and fairly tight for those I am trading. If you see this happening a lot switch providers.