@ Hardy:
1. Perhaps what you mean by market line is the market price or the current market price.
If the order you open is buy, then to close (as a profit target price) is the price in the bid column, conversely if the order you open is sell, then to close is the price in the ask column. While the price displayed on the chart (market price or market line) is the bid price.
Perhaps you opened a sell order where to close you must refer to the ask price.
For example: You sell EUR/USD at a price of 1.1800 with a profit target at a price of 1.1750. When the price on the chart (market line) shows 1.1750 (bid price), your order has not been closed because the ask price is still above 1.1750, maybe 1.1751 or 1.1752, depending on the broker's spread.
Let's say the spread is 2 pips, if later the price on the chart is 1.1748 (bid price) then your sell order will be closed because at that time the ask price has reached 1.1750 (your profit target).
2. Perhaps what you mean by the demand line is Ask Line, and the market line is the bid price. Do you mean that they are parallel, they overlap? If they overlap then the spread is zero, and that is impossible. Maybe they look overlapping but the spread still exists even though it is small.
If you are not trading with a broker with a fixed spread, then the spread can indeed change depending on market conditions. When market volume is high, the spread usually widens. Also when there is a release of high-impact fundamental data where trading volume increases sharply at that time.
In standard trading calculations, the spread is the difference between the ask price and the bid price, so the spread is always positive, never negative.