If you are are talking about a romantic relationship, the answer is no. By definition, a straight girl does not want a romantic relationship with another girl.
That means that if you draw a graph of the relationship, you will not get a straight line.
Linear = Straight line! So, a graph of a linear relationship is a straight line.
The answer depends on what you mean by "A".
No. Homosexuality is a state of human sexuallity, not a state of relationship. To be in a gay relationship would definitely imply you are homosexual, but to be homosexual would not imply you are in a relationship. Compare it with heterosexuality - you can be straight, but not in a straight relationship.
it means you'e hetero
No.
yes unless he had cheated on the previous girlfriend with the current one.
it is a positive relationship
It is no different than the relationship between straight parents and their children.
A direct relationship if the slope of the line is positive. An inverse relationship if the slope of the line is negative.
By definition, if you graph the relationship between two variables and the result is a straight line (of whatever slope) that is a linear relationship. If it is a curve, rather than a straight line, then it is not linear.