My 2.5 yr old has not grown much in the past 6 mon. What kind of trend line on the growth chart should trigger a worry? What could be the cause?

Growth curve. Will be plotted every time you visit your pediatrician, the rate of growth slows alot after the second year compared to the first year of life which can seem concerning, ask your doc to show you his growth curve.
Growth deceleration. Crossing height percentiles is not normal. There can be periods of time with slower growth but these generally are not prolonged over 6 months, . Be sure the measurements were accurate. It's hard to measure height at that age.