private function computeLinear( $i, $d ) {
		$q = $this->heights[$i];
		$n = $this->positions[$i];
		return ( $q + $d *
			( $this->heights[$i + $d] - $q ) /
			( $this->positions[$i + $d] - $n )
		);
	}
private function computeLinear( $i, $d ) {
		$q = $this->heights[$i];
		$n = $this->positions[$i];
		return ( $q + $d *
			( $this->heights[$i + $d] - $q ) /
			( $this->positions[$i + $d] - $n )
		);
	}
private function computeLinear ( $ i , $ d ) { $ q = $ this -> heights [ $ i ] ; $ n = $ this -> positions [ $ i ] ; return ( $ q + $ d * ( $ this -> heights [ $ i + $ d ] - $ q ) / ( $ this -> positions [ $ i + $ d ] - $ n ) ) ; }
Linear formula to predict ideal position of a marker.

@param int $i index of marker to adjust
@param int $d always -1 or 1
@return float ideal height of marker
Linear formula to predict ideal position of a marker .