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I can make a sine wave travel parallel to the $x$ and $y$ axis of a cartesian plane, however I have yet to find any formula that allows one to change the gradient of the sine wave. How does one do that? I have tried the following, and it is not what I am after:

$f(x)=\sin(x)+x$

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    How about $x=\sin y$? – 2017-02-28
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    nay, that makes the wave paralell to the y axis, thanks though – 2017-02-28
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    So what do you mean by "parallel to the x and y axis"? Do you mean at $45^\circ$ outwards? – 2017-02-28
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    oooh, soz, quite right, i meant to say i can only make the wave parallel to the x or y axis, how does one change the gradient of the wave, as in make it not paralell to the x or y axis – 2017-02-28
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    Oh, right, it actually says so in the question. I apparently can't read today. – 2017-02-28

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In linear algebra you learn to rotate the plane. If you want to rotate some geometrical structure defined by its coordinates (such as the graph of a function, defined by the points $(x, f(x))$, or a figure defined by an equation, or just a list of points) counterclockwise by an angle of $\theta$, you replace $x$ by $x\cos\theta + y\sin\theta$ and you replace $y$ with $y\cos\theta - x\sin\theta$. This means that the formula for a rotated sine wave (given originally by $y = \sin(x)$) is $$ y\cos\theta - x\sin\theta = \sin\left(x\cos\theta + y\sin\theta\right) $$ If the axis of the rotated sine wave is closer to the $x$-axis than to the $y$-axis (for instance is $\theta$ is between $\pm 45^\circ$), then this can theoretically be rearranged to a function $y = f(x)$, but I have no idea how it would be done. Similarily, if the new axis is more aligned with the $y$-axis, the above expression can be rearranged to a function $x = f(y)$, but I still do not know how.

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    thanks alot for that, this makes exaclty what i wanted. – 2017-03-01
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When dealing with transformations the only way to change gradient is by using a stretch. Either f((1/scale)x) for a stretch in the x direction or (1/scale)f(x) for a stretch in the y direction. However this does not directly change the gradient of sine, it will also affect your range and domain.

Let's say you have sin(2x) a stretch of a half in x direction, differentiating this gives us 2cos(2x), which is 2 times the original gradient showing that the gradient has double excluding turning points.

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    While this is a good answer to a different question, this is not what the asker wants. He wants a formula for a _rotated_ graph of a sine wave. Not compressed / stretched like you get when changing the argument to $ax$, not skewed like you get by adding $x$ outside the sine. – 2017-02-28
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    My apologies. A misunderstanding has occurred. – 2017-02-28