The phrase "a smooth line" is correct and usable in written English.
It can be used to describe a line that is even, continuous, or free of bumps and irregularities, often in contexts related to art, design, or geometry.
Example: "The artist skillfully created a smooth line that flowed effortlessly across the canvas."
Alternatives: "a sleek line" or "an even line".
Human behavior never follows a smooth line".
Keep the tops slim and tucked in, so that you have a smooth line.
At Chanel, while the hapless artist tried to draw a smooth line around her lips, a shopper once jabbered about her efforts to get off psychotropic drugs.
There's even a section on arrest: "Sometimes even guys with cool sideburns and a smooth line of patter get arrested for sexual harassment and are issued summonses".
The T-shirt bras have darts only on the bottom half of the cup, so that the part which touches the garment above gives a smooth line.
From the lowest 10th to the ninth decile, the difference in income levels rises in a smooth line, but between the ninth and 10th deciles incomes rise by nearly 70%.
If you'll be wearing the bathrobe over pajamas, do you want a smooth lining that slips well?
In a photograph of the coach in the Sydney sesquicentenary parade, the roof appears to have a smoother line at the top, with the landau bars less prominent and some edge trim present.
Power graphs are plotted as a smoothed line based on 51 different values of ρ 1, and for each of these values of the graph, 1000 repetitions of a test of independence are generated.
The size distribution of MNBs obtained from TEM images of replica samples prepared just after aeration (solid circles with arbitrary unit, n = 114) is similar to that measured by a dynamic light scattering method (open diamonds with error bars and a smoothed line), which was reproduced from Ushikubo et al. [19].
To fit the data, a smoothed line, represented by the logarithmic trend, was also added.
