Does the demand curve have to be a straight line?

The demand curve can be a straight line or a curve, of course, it is a curve in most cases. On the coordinate axis, if the abscissa is price and the ordinate is quantity, then the demand curve will generally tilt to the lower right, which is also in line with our common sense, that is, the lower the price, the greater the demand. But it is different from a special case that the demand curve is perpendicular to the X axis or the Y axis, that is, the demand remains unchanged no matter how the price changes or the price remains unchanged no matter how the demand changes.

As for when the straight line is a curve, it is necessary to analyze the specific situation, that is, when the data you collected are fitted into a straight line, the demand curve will be in place, and vice versa. Generally speaking, when some calculations are inaccurate, it is customary to use straight lines, because it is much simpler to express them with straight lines.

I hope my answer will satisfy you.