<p>The default chart object in the SQL Server (2005) Business Intelligence Development Studio doesn't seem to some have formatting options like :</p>

<ol>
<li>specifying the text direction of labels in the x and y axis</li>
<li>adding a trendline to a bar chart</li>
<li>arbitrarily resizing items in a chart - for example, if I resize the chart object, everything gets resized accordingly but I can't keep the size of the chart the same while extending the area of the legend for instance.</li>
<li>multiline chart labels</li>
</ol>

<p>So what I want to know is </p>

<ol>
<li>is there any easy answer to the formatting problems mentioned above?</li>
<li>what websites/books/resources/examples would you recommend I look
    into for extending the functionality of the chart object?</li>
</ol>

yes you can specify the text direction of labels in the x and y axis
Go to chart properties and in the tab x and y axis enter the chart title and in the title align use the combination like left/right/center align.
you can change the legend line go to the chart properties click legend tab inside this
there is an option for "display legend inside plot area" and you can include the trendline there
you can use multiline text labels when the text limits extends