/**
  * Returns the difference between the first and the last X sample.
  * Must not be called on an empty object.
  */
 double GetDeltaX() const {
   return GetLastX() - GetFirstX();
 }
 /**
  * Returns the difference between the first and the last X sample.
  * Must not be called on an empty object.
  */
 fixed GetDeltaX() const {
   return GetLastX() - GetFirstX();
 }