Interpolate without having negative values in python
<p>I've been trying to create a smooth line from these values but I can't have negative values in my result. So far all the methods I tried do give negative values. Would love some help.</p>

<pre><code>import matplotlib.pyplot as plt
from scipy.interpolate import UnivariateSpline
import numpy as np
y = np.asarray([0,5,80,10,1,10,40,30,80,5,0])
x = np.arange(len(y))

plt.plot(x, y, 'r', ms=5)
spl = UnivariateSpline(x, y)
xs = np.linspace(0,len(y)-1, 1000)
spl.set_smoothing_factor(2)

plt.plot(xs, spl(xs), 'g', lw=3)
plt.show()
</code></pre>

<p><a href="https://i.stack.imgur.com/RY5hW.png" rel="nofollow"><img src="https://i.stack.imgur.com/RY5hW.png" alt="enter image description here"></a></p>

<p>Spline fitting is known to overshoot. You seem to be looking for one of the so-called <em>monotonic</em> interpolators. For instance,</p>

<pre><code>In [10]: from scipy.interpolate import pchip

In [11]: pch = pchip(x, y)
</code></pre>

<p>produces</p>

<pre><code>In [12]: xx = np.linspace(x[0], x[-1], 101)

In [13]: plt.plot(x, y, 'ro', label='points')
Out[13]: [&lt;matplotlib.lines.Line2D at 0x7fce0a7fe390&gt;]

In [14]: plt.plot(xx, pch(xx), 'g-', label='pchip')
Out[14]: [&lt;matplotlib.lines.Line2D at 0x7fce0a834b10&gt;]
</code></pre>

<p><a href="https://i.stack.imgur.com/xuYMR.png"><img src="https://i.stack.imgur.com/xuYMR.png" alt="enter image description here"></a></p>

python numpy matplotlib scipy interpolation