I do not really understand what your question is. You need to explain what exactly you want to do.

You can make line art in Microsoft PowerPoint - that is probably one of the easiest softwares to work with (also a little bit limited - but probably good enough for many scientific line-art purposes).

Could you clarify your question? Do you mean line art (for illustration of shapes, anatomical structures, animals, etc.) OR do you mean how to illustrate (using a line) a relationship between two factors?

Given the description of your question, I think you mean how to illustrate a relationship between two factors on a scatterplot (i.e. "the shape of data"). The shape of data can only be determined by a best-fit model. It is common for some researchers to first plot the relationship of their data and then model these data. This is especially common in data with some level of clustering; that is, a scatterplot that contains some proportion of non-independent data points. In this case, the data must first be modeled to account for clustering. Then you'll find a way to plot this relationship.

Excel (or simple curve-fitting in other programs) will allow fitting basic lines, but it is critical to first understand if such relationships (polynomial, linear, etc.) fit the data.

Thanks for your answers

My mean from these two graphs is illustrated in two below figures:

I see, then never mind my answer, you should not use PowerPoint to draw graph trendlines. "Line art" is a descriptive picture of something in a paper, i.e. something like this: http://kaevia.deviantart.com/art/Bird-Scientific-Line-Drawing-274049332