The stock has delivered superior returns in the past 19 years.Medical device specialist DexCom (DXCM -0.40%) is going through a rough patch. The company's shares dropped off a cliff following its second-quarter earnings report. However, zooming out helps put things in perspective. The healthcare company has generally delivered returns that are well above average. Can DexCom continue doing that long enough to be a millionaire-maker stock for investors who get in today? Let's find out.What DexCom doesDexCom i ...