Implement STL-like interface for plotting generic data structures

[Hs293Go]

This PR is motivated by the desire to plot data in generic data structures without the boilerplate of copying data into std::vectors before each plot. (I just had to plot a 101385-element Eigen::VectorXd and the copy was NOT fun).

The solution adopted is to follow the example of the STL and define the graph data by a pair of templated input iterators. For ease of use, the interface is designed to mimic the STL in the following manner.

std::transform(a.begin(), a.end(), b.begin(), binaryFunctor);
plt::stl::plot(a.begin(), a.end(), b.begin(), "ro");

only 2D lineplots and scatter plots are addressed for the time being.

In due diligence, the examples stl_containers.cpp and eigen_objects.cpp and a section reviewing this interface in the README are added.

I think the main sticking points regarding this PR are

1. If the STL-like interface looks out of place among functions mimicking matplotlib's interface
2. The mechanism handling Contiguous and Non-Contiguous containers is clunky.

I suppose C++20 <ranges> is the silver bullet to resolve point 1, and C++20 contiguous_iterator concept deals with point 2, but this is the most elegant solution I have for now. If there's anything I can to do better before this PR can be merged, let me know!

[amadeus84]

I came across the same problem, I have data in c-style arrays

doble* x, * y:
size_t n; // same for x and y

and to pass it to plt::plot() I'd have to copy it into a vector first. Ranges (C++20) work! I tried. But perhaps the simplest solution I came up with was

auto xx = std::span(x, n);
auto yy = std:;span(y, n);
plt::plot(xx, yy, "");

(of course, you can put the span directly into plot(), no need to have xx and yy). And, of course, this works also when y has length a multiple of lenght of x:

double* x, * y;
size_t nx=100, ny = 5*nx;

for (size_t offset = 0; offset < ny; offset += nx)
plt::plot(std::span(x, nx), std::span(y+offset, nx), "");

So far so good, and if this is available in C++20, I don't see why we shouldn't use it. In fact, the whole variadic template part of matplotlibcpp looks like was specifically done with ranges in mind - all that's needed is begin(), end(), etc. i.e all the things a range would have.

The problem though, is that for types other than vector, matplotlibcpp ends up calling plot_impl(), which then copies the inputs to PyObject arrays. It defeats the purpose of using span / ranges in the first place.

Perhaps we can have a template specialization of plot (or plot_impl) for contiguous ranges, that would not iterate and copy the input data to PyObject arrays, but rather call PyArray_SimpleNewFromData(), like plot(vector) does, via get_array(). According to https://numpy.org/devdocs/reference/c-api/array.html PyArray_SimpleNewFromData() does not copy the data.

I'm experimenting with this now.

[Hs293Go]

@amadeus84 I like your ideas! A potential difficulty may arise from the fact that plotting is always with a pair of ranges, and some inspiration might be taken from zip_view (currently in range-v3 only) to design concepts that ensure compatibility between the pair of ranges.

Alas, matplotlibcpp seem to be unmaintained for almost a year already. I've switched to the technique of using pybind11 to embed the python interpreter in C++ code and plot using the native matplotlib API. I just made public a demo of the code I've been sharing around: https://github.com/Hs293Go/demo_pybind11_matplotlib

[amadeus84]

@amadeus84 I like your ideas! A potential difficulty may arise from the fact that plotting is always with a pair of ranges, and some inspiration might be taken from zip_view (currently in range-v3 only) to design concepts that ensure compatibility between the pair of ranges.

Alas, matplotlibcpp seem to be unmaintained for almost a year already. I've switched to the technique of using pybind11 to embed the python interpreter in C++ code and plot using the native matplotlib API. I just made public a demo of the code I've been sharing around: https://github.com/Hs293Go/demo_pybind11_matplotlib

I now have support for ranges in my repository: https://github.com/amadeus84/matplotlib-cpp