calculate_patlak_x
=============================================================

.. py:function:: petpal.kinetic_modeling.graphical_analysis.calculate_patlak_x(tac_times: numpy.ndarray, tac_vals: numpy.ndarray) -> numpy.ndarray

   Calculates the x-variable in Patlak analysis
   :math:`\left(\frac{\int_{0}^{T}f(t)\mathrm{d}t}{f(T)}\right)`.

   Patlak-Gjedde analysis is a linearization of the 2-TCM with irreversible uptake in the second
   compartment. Therefore, we essentially have to fit a line to some data :math:`y = mx+b`. This
   function calculates the :math:`x` variable for Patlak analysis where,

   .. math::

       x = \frac{\int_{0}^{T} C_\mathrm{P}(t) \mathrm{d}t}{C_\mathrm{P}(t)},

   where further :math:`C_\mathrm{P}` is usually the plasma TAC.

   :param tac_times: Array containing the sampled times.
   :type tac_times: np.ndarray
   :param tac_vals: Array for activity values at the sampled times.
   :type tac_vals: np.ndarray

   :returns: *(np.ndarray)* -- Patlak x-variable. Cumulative integral of activity divided by activity.